Exclude isc_mem_isovermem from ThreadSanitizer

The .is_overmem member of isc_mem_t structure is intentionally accessed
unlocked as 100% accuracy isn't necessary here.

Without the attribute, following TSAN warning would show up:

    WARNING: ThreadSanitizer: data race
      Write of size 1 at 0x000000000001 by thread T1 (mutexes: write M1, write M2):
	#0 isc___mem_put lib/isc/mem.c:1119:19
	#1 isc__mem_put lib/isc/mem.c:2439:2
	#2 dns_rdataslab_fromrdataset lib/dns/rdataslab.c:327:2
	#3 addrdataset lib/dns/rbtdb.c:6761:11
	#4 dns_db_addrdataset lib/dns/db.c:719:10
	#5 cache_name lib/dns/resolver.c:6538:13
	#6 cache_message lib/dns/resolver.c:6628:14
	#7 resquery_response lib/dns/resolver.c:7883:13
	#8 dispatch lib/isc/task.c:1152:7
	#9 run lib/isc/task.c:1344:2

      Previous read of size 1 at 0x000000000001 by thread T2 (mutexes: write M3):
	#0 isc_mem_isovermem lib/isc/mem.c:1553:15
	#1 addrdataset lib/dns/rbtdb.c:6866:25
	#2 dns_db_addrdataset lib/dns/db.c:719:10
	#3 addoptout lib/dns/ncache.c:281:10
	#4 dns_ncache_add lib/dns/ncache.c:101:10
	#5 ncache_adderesult lib/dns/resolver.c:6668:12
	#6 ncache_message lib/dns/resolver.c:6845:11
	#7 rctx_ncache lib/dns/resolver.c:9174:11
	#8 resquery_response lib/dns/resolver.c:7894:2
	#9 dispatch lib/isc/task.c:1152:7
	#10 run lib/isc/task.c:1344:2

      Location is heap block of size 328 at 0x000000000020 allocated by thread T3:
	#0 malloc <null>
	#1 default_memalloc lib/isc/mem.c:713:8
	#2 mem_create lib/isc/mem.c:763:8
	#3 isc_mem_create lib/isc/mem.c:2425:2
	#4 configure_view bin/named/server.c:4494:4
	#5 load_configuration bin/named/server.c:9062:3
	#6 run_server bin/named/server.c:9771:2
	#7 dispatch lib/isc/task.c:1152:7
	#8 run lib/isc/task.c:1344:2

    [...]

    SUMMARY: ThreadSanitizer: data race lib/isc/mem.c:1119:19 in isc___mem_put
75 jobs for !4140 with 2163-threadsanitizer-data-race-lib-isc-mem-c-1119-19-in-isc___mem_put in 27 minutes and 25 seconds (queued for 8 seconds)
latest detached
Status Job ID Name Coverage
  Autoconf
passed #1163263
amd64 linux
autoreconf

00:00:18

 
  Precheck
passed #1163265
amd64 linux
clang-format

00:00:17

passed #1163266
amd64 linux
coccinelle

00:03:29

passed #1163267
amd64 linux
danger

00:00:13

passed #1163264
amd64 linux
misc

00:00:12

passed #1163268
amd64 linux
tarball-create

00:01:34

 
  Build
passed #1163281
amd64 linux
clang:asan

00:02:06

passed #1163285
amd64 linux
clang:buster:amd64

00:01:06

passed #1163288
amd64 libvirt
clang:freebsd11:amd64

00:01:17

passed #1163289
amd64 libvirt
clang:freebsd12:amd64

00:01:18

passed #1163290
amd64 libvirt
clang:openbsd:amd64

00:02:56

passed #1163283
amd64 linux
clang:tsan

00:01:19

passed #1163269
amd64 linux
gcc:alpine3.12:amd64

00:01:00

passed #1163280
amd64 linux
gcc:asan

00:01:43

passed #1163273
amd64 linux
gcc:buster:amd64

00:00:58

passed #1163270
amd64 linux
gcc:centos7:amd64

00:00:52

passed #1163271
amd64 linux
gcc:centos8:amd64

00:00:58

passed #1163279
amd64 linux
gcc:focal:amd64

00:00:59

passed #1163284
amd64 linux
gcc:mutexatomics

00:00:52

passed #1163275
amd64 linux
gcc:out-of-tree

00:01:01

passed #1163274
amd64 linux
gcc:sid:amd64

00:01:13

passed #1163276
i386 linux
gcc:sid:i386

00:01:01

passed #1163286
amd64 linux
gcc:softhsm2.4

00:00:52

passed #1163287
amd64 linux
gcc:softhsm2.6

00:00:58

passed #1163272
amd64 linux
gcc:stretch:amd64

00:01:03

passed #1163282
amd64 linux
gcc:tsan

00:01:01

passed #1163277
amd64 linux
gcc:tumbleweed:amd64

00:01:04

passed #1163278
amd64 linux
gcc:xenial:amd64

00:01:01

passed #1163291
amd64 windows
msvc:windows:amd64

00:00:43

 
  Unit
passed #1163303
amd64 linux
unit:clang:asan

00:01:15

passed #1163307
amd64 linux
unit:clang:buster:amd64

00:00:57

passed #1163310
amd64 libvirt
unit:clang:freebsd11:amd64

00:01:26

passed #1163311
amd64 libvirt
unit:clang:freebsd12:amd64

00:01:14

passed #1163305
amd64 linux
unit:clang:tsan

00:03:00

passed #1163292
amd64 linux
unit:gcc:alpine3.12:amd64

00:00:58

passed #1163302
amd64 linux
unit:gcc:asan

00:01:13

passed #1163296
amd64 linux
unit:gcc:buster:amd64

00:00:59

passed #1163293
amd64 linux
unit:gcc:centos7:amd64

00:01:06

passed #1163294
amd64 linux
unit:gcc:centos8:amd64

00:00:59

passed #1163301
amd64 linux
unit:gcc:focal:amd64

00:00:57

passed #1163306
amd64 linux
unit:gcc:mutexatomics

00:01:00

passed #1163297
amd64 linux
unit:gcc:sid:amd64

00:00:59

passed #1163298
i386 linux
unit:gcc:sid:i386

00:01:07

passed #1163308
amd64 linux
unit:gcc:softhsm2.4

00:01:00

passed #1163309
amd64 linux
unit:gcc:softhsm2.6

00:00:59

passed #1163295
amd64 linux
unit:gcc:stretch:amd64

00:00:57

passed #1163304
amd64 linux
unit:gcc:tsan

00:03:49

passed #1163299
amd64 linux
unit:gcc:tumbleweed:amd64

00:00:57

passed #1163300
amd64 linux
unit:gcc:xenial:amd64

00:01:03

 
  System
passed #1163323
amd64 linux
system:clang:asan

00:11:47

passed #1163327
amd64 linux
system:clang:buster:amd64

00:09:57

passed #1163330
amd64 libvirt
system:clang:freebsd11:amd64

00:10:46

passed #1163444
libvirt amd64
system:clang:freebsd12:amd64

00:10:35

failed #1163325
amd64 linux allowed to fail
system:clang:tsan

00:14:07

passed #1163312
amd64 linux
system:gcc:alpine3.12:amd64

00:11:45

passed #1163322
amd64 linux
system:gcc:asan

00:11:51

passed #1163316
amd64 linux
system:gcc:buster:amd64

00:10:30

passed #1163313
amd64 linux
system:gcc:centos7:amd64

00:11:02

passed #1163314
amd64 linux
system:gcc:centos8:amd64

00:11:59

passed #1163321
amd64 linux
system:gcc:focal:amd64

00:10:49

passed #1163326
amd64 linux
system:gcc:mutexatomics

00:10:30

passed #1163317
amd64 linux
system:gcc:sid:amd64

00:10:09

passed #1163318
i386 linux
system:gcc:sid:i386

00:10:53

passed #1163328
amd64 linux
system:gcc:softhsm2.4

00:10:30

passed #1163329
amd64 linux
system:gcc:softhsm2.6

00:11:27

passed #1163315
amd64 linux
system:gcc:stretch:amd64

00:10:16

failed #1163324
amd64 linux allowed to fail
system:gcc:tsan

00:14:27

passed #1163319
amd64 linux
system:gcc:tumbleweed:amd64

00:10:27

passed #1163320
amd64 linux
system:gcc:xenial:amd64

00:09:43

failed #1163331
amd64 libvirt
system:clang:freebsd12:amd64

00:09:56

 
  Docs
passed #1163332
amd64 linux
docs

00:00:36

 
  Postcheck
passed #1163336
amd64 linux
cppcheck

00:00:46

passed #1163333
amd64 linux
flake8

00:00:21

passed #1163334
amd64 linux
pylint

00:00:24

passed #1163335
amd64 linux
scan-build

00:04:18

 
Name Stage Failure
failed
system:gcc:tsan System
E:shutdown:2020-09-17T14:08:20+0000
FAIL shutdown (exit status: 1)

$ find bin -name 'tsan.*' -exec python3 util/parse_tsan.py {} \;
Uploading artifacts for failed job
Uploading artifacts...
untracked: found 9918 files

Uploading artifacts as "archive" to coordinator... ok
id=1163324 responseStatus=201 Created token=sht-xbR8
ERROR: Job failed: exit code 1
failed
system:clang:tsan System
E:shutdown:2020-09-17T14:07:58+0000
FAIL shutdown (exit status: 1)

$ find bin -name 'tsan.*' -exec python3 util/parse_tsan.py {} \;
Uploading artifacts for failed job
Uploading artifacts...
untracked: found 9728 files

Uploading artifacts as "archive" to coordinator... ok
id=1163325 responseStatus=201 Created token=yQbCxCtj
ERROR: Job failed: exit code 1