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

lib/isc/mem.c

@@ -1539,7 +1539,7 @@ isc_mem_setwater(isc_mem_t *ctx0, isc_mem_water_t water, void *water_arg,
 	}
 }
 
-bool
+ISC_NO_SANITIZE_THREAD bool
 isc_mem_isovermem(isc_mem_t *ctx0) {
 	REQUIRE(VALID_CONTEXT(ctx0));