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)

Closes #2163 (closed)