This is a reland of commit abcb6bb8.
The data race is fixed by using atomic operations.

Original change's description:
> [heap] Avoid dynamic updates of FLAG_gc_interval
>
> Flags will be protected from updates after V8 initialization (in the
> future). This CL avoids any updates of the --gc-interval flag during
> runtime, and instead updates a static field on the HeapAllocator
> directly.
>
> R=mlippautz@chromium.org
>
> Bug: v8:12887
> Change-Id: I17a495cae50a46d59a8159c6ece1558d4d61b949
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3687691
> Commit-Queue: Clemens Backes <clemensb@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#80998}

Bug: v8:12887
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Change-Id: Ib5b537500413a627d9b2509354d20906e0474d8e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3695380Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81008}