MutatorMarkerRace_DetectInTLAB test compile error on GCC

[liamappelbe]

The GCC bot doesn't like the new MutatorMarkerRace_DetectInTLAB test added in https://dart-review.googlesource.com/c/sdk/+/390263

Logs: https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8733223566365977505/+/u/build_dart_debug_ia32/stdout

[dart-github-bot]

Summary: The MutatorMarkerRace_DetectInTLAB test, introduced in CL 390263, fails to compile on GCC. The GCC bot reports a compilation error, and the logs provide details of the failure.

[a-siva]

https://dart-review.googlesource.com/c/sdk/+/392063 should fix this issue.