[test] Synthetic versions of the mutator-marker race.

TEST=vm/cc/MutatorMarkerRace_*
Bug: https://github.com/dart-lang/sdk/issues/56895
Change-Id: I3e3f25b2c43aec09a409377f77c6d2fec287bccb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/390263
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/vm/class_table.cc

@@ -85,6 +85,10 @@ void ClassTable::Register(const Class& cls) {
         classes_.GetColumn<kClassIndex>());
     UpdateCachedAllocationTracingStateTablePointer();
   } else {
+    // GCC warns that TSAN doesn't understand thread fences.
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic ignored "-Wtsan"
+#endif
     std::atomic_thread_fence(std::memory_order_release);
   }
 }