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

(cherry picked from commit 0110d1ab)
82 jobs for !4144 with 2163-threadsanitizer-data-race-lib-isc-mem-c-1119-19-in-isc___mem_put-v9_11 in 32 minutes and 2 seconds (queued for 11 seconds)
latest detached
Status Job ID Name Coverage
  Precheck
passed #1163633
amd64 linux
autoreconf

00:00:15

passed #1163635
amd64 linux
clang-format

00:00:08

passed #1163636
amd64 linux
coccinelle

00:01:00

passed #1163868
linux amd64
danger

00:00:15

passed #1163634
amd64 linux
misc

00:00:13

failed #1163637
amd64 linux
danger

00:00:13

 
  Build
passed #1163652
amd64 linux
clang:asan

00:04:33

passed #1163656
amd64 linux
clang:buster:amd64

00:01:53

passed #1163658
amd64 libvirt
clang:freebsd11:amd64

00:02:03

passed #1163659
amd64 libvirt
clang:freebsd12:amd64

00:01:45

passed #1163660
amd64 libvirt
clang:openbsd:amd64

00:04:21

passed #1163654
amd64 linux
clang:tsan

00:02:23

passed #1163638
amd64 linux
gcc:alpine3.12:amd64

00:01:48

passed #1163651
amd64 linux
gcc:asan

00:03:58

passed #1163643
amd64 linux
gcc:buster:amd64

00:01:43

passed #1163639
amd64 linux
gcc:centos6:amd64

00:01:32

passed #1163640
amd64 linux
gcc:centos7:amd64

00:01:33

passed #1163641
amd64 linux
gcc:centos8:amd64

00:01:37

passed #1163648
amd64 linux
gcc:fedora32:amd64

00:01:59

passed #1163650
amd64 linux
gcc:focal:amd64

00:01:46

passed #1163655
amd64 linux
gcc:mutexatomics

00:02:10

passed #1163645
amd64 linux
gcc:out-of-tree

00:01:44

passed #1163657
amd64 linux
gcc:pkcs11

00:01:42

passed #1163644
amd64 linux
gcc:sid:amd64

00:02:39

passed #1163646
i386 linux
gcc:sid:i386

00:01:51

passed #1163642
amd64 linux
gcc:stretch:amd64

00:01:55

passed #1163653
amd64 linux
gcc:tsan

00:02:04

passed #1163647
amd64 linux
gcc:tumbleweed:amd64

00:01:43

passed #1163649
amd64 linux
gcc:xenial:amd64

00:02:00

passed #1163662
amd64 windows
msvc:windows:amd64

00:01:03

passed #1163661
amd64 linux
nolibtool:sid:amd64

00:02:14

 
  Unit
passed #1163676
amd64 linux
unit:clang:asan

00:01:14

passed #1163680
amd64 linux
unit:clang:buster:amd64

00:00:28

passed #1163682
amd64 libvirt
unit:clang:freebsd11:amd64

00:01:10

passed #1163683
amd64 libvirt
unit:clang:freebsd12:amd64

00:02:28

passed #1163678
amd64 linux
unit:clang:tsan

00:04:10

passed #1163663
amd64 linux
unit:gcc:alpine3.12:amd64

00:00:32

passed #1163675
amd64 linux
unit:gcc:asan

00:01:14

passed #1163668
amd64 linux
unit:gcc:buster:amd64

00:00:31

passed #1163664
amd64 linux
unit:gcc:centos6:amd64

00:00:34

passed #1163665
amd64 linux
unit:gcc:centos7:amd64

00:00:33

passed #1163666
amd64 linux
unit:gcc:centos8:amd64

00:00:31

passed #1163672
amd64 linux
unit:gcc:fedora32:amd64

00:00:31

passed #1163674
amd64 linux
unit:gcc:focal:amd64

00:00:33

passed #1163679
amd64 linux
unit:gcc:mutexatomics

00:00:36

passed #1163681
amd64 linux
unit:gcc:pkcs11

00:00:34

passed #1163669
amd64 linux
unit:gcc:sid:amd64

00:00:34

passed #1163670
i386 linux
unit:gcc:sid:i386

00:00:28

passed #1163667
amd64 linux
unit:gcc:stretch:amd64

00:00:33

passed #1163677
amd64 linux
unit:gcc:tsan

00:05:12

passed #1163671
amd64 linux
unit:gcc:tumbleweed:amd64

00:00:31

passed #1163673
amd64 linux
unit:gcc:xenial:amd64

00:00:32

passed #1163684
amd64 linux
unit:nolibtool:sid:amd64

00:00:45

 
  System
passed #1163698
amd64 linux
system:clang:asan

00:09:02

passed #1163702
amd64 linux
system:clang:buster:amd64

00:07:05

passed #1165064
libvirt amd64
system:clang:freebsd11:amd64

00:08:01

passed #1163705
amd64 libvirt
system:clang:freebsd12:amd64

00:08:08

passed #1163700
amd64 linux
system:clang:tsan

00:11:03

passed #1163685
amd64 linux
system:gcc:alpine3.12:amd64

00:07:37

passed #1163697
amd64 linux
system:gcc:asan

00:09:33

passed #1163690
amd64 linux
system:gcc:buster:amd64

00:07:25

passed #1163686
amd64 linux
system:gcc:centos6:amd64

00:07:16

passed #1163687
amd64 linux
system:gcc:centos7:amd64

00:07:25

passed #1163688
amd64 linux
system:gcc:centos8:amd64

00:07:09

passed #1163694
amd64 linux
system:gcc:fedora32:amd64

00:07:17

passed #1163696
amd64 linux
system:gcc:focal:amd64

00:07:10

passed #1165063
linux amd64
system:gcc:mutexatomics

00:07:30

passed #1163703
amd64 linux
system:gcc:pkcs11

00:08:06

passed #1163691
amd64 linux
system:gcc:sid:amd64

00:06:58

passed #1163692
i386 linux
system:gcc:sid:i386

00:08:05

passed #1163689
amd64 linux
system:gcc:stretch:amd64

00:07:08

passed #1163699
amd64 linux
system:gcc:tsan

00:12:09

passed #1163693
amd64 linux
system:gcc:tumbleweed:amd64

00:07:11

passed #1163695
amd64 linux
system:gcc:xenial:amd64

00:06:39

passed #1163706
amd64 linux
system:nolibtool:sid:amd64

00:07:34

failed #1163704
amd64 libvirt
system:clang:freebsd11:amd64

01:00:00

failed #1163701
amd64 linux
system:gcc:mutexatomics

00:07:16

 
  Docs
passed #1163707
amd64 linux
docs

00:00:26

 
  Postcheck
passed #1163711
amd64 linux
cppcheck

00:00:24

passed #1163708
amd64 linux
flake8

00:00:19

passed #1163709
amd64 linux
pylint

00:00:21

passed #1163710
amd64 linux
scan-build

00:08:30