CBD-5603: Add leak:__cxa_thread_atexit to lsan.suppressions

When moving from ubuntu to centos based workers, new memory leaks
emerged in some tests:

  ep-engine_ep_unit_tests.RocksFullOrValue/DurabilityWarmupTest:
    Direct leak of 48 byte(s) in 2 object(s) allocated from:
      #0 0x6e2e27 in operator new(unsigned long, std::nothrow_t const&) /tmp/llvm-project/compiler-rt/lib/asan/asan_new_delete.cc:105:3
      #1 0x7efc40b38fcd in __cxa_thread_atexit /tmp/deploy/objdir/../gcc-10.2.0/libstdc++-v3/libsupc++/atexit_thread.cc:146:37
    SUMMARY: AddressSanitizer: 48 byte(s) leaked in 2 allocation(s).

  ep_testsuite_checkpoint.value_eviction.rocksdb:
    Direct leak of 288 byte(s) in 12 object(s) allocated from:
      #0 0x571427 in operator new(unsigned long, std::nothrow_t const&) /tmp/llvm-project/compiler-rt/lib/asan/asan_new_delete.cc:105:3
      #1 0x7faea5f13fcd in __cxa_thread_atexit /tmp/deploy/objdir/../gcc-10.2.0/libstdc++-v3/libsupc++/atexit_thread.cc:146:37
    SUMMARY: AddressSanitizer: 288 byte(s) leaked in 12 allocation(s).

  ep_testsuite_checkpoint.full_eviction.rocksdb:
    Direct leak of 360 byte(s) in 15 object(s) allocated from:
      #0 0x571427 in operator new(unsigned long, std::nothrow_t const&) /tmp/llvm-project/compiler-rt/lib/asan/asan_new_delete.cc:105:3
      #1 0x7f3810c11fcd in __cxa_thread_atexit /tmp/deploy/objdir/../gcc-10.2.0/libstdc++-v3/libsupc++/atexit_thread.cc:146:37
    SUMMARY: AddressSanitizer: 360 byte(s) leaked in 15 allocation(s).

This appears to perhaps be an issue with rocksdb (similar errors in
facebook/rocksdb#5931) or maybe libstdc++
itself, but given we don't ship rocksdb, this change simply adds
__cxa_thread_atexit to the suppressions file.

Change-Id: I615822181c54aa7fc3b0c0db00d7b70008323008
Reviewed-on: https://review.couchbase.org/c/tlm/+/196843
Tested-by: Blair Watt <blair.watt@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>

lsan.suppressions

@@ -7,4 +7,8 @@
 
 # MB-41894: v8 8.3 with pointer compression disabled has some read-only memory
 # Supressing isolate leaks until we enable pointer compression
-leak:v8::Isolate::New
+leak:v8::Isolate::New
+
+# CBD-5603: RocksDB memory leaks on single-linux workers
+# See also - https://github.com/facebook/rocksdb/issues/5931
+leak:__cxa_thread_atexit