BasicMemoryChunk flags should only be mutated from the main thread
when no concurrent task can access them. For that purpose it is enough
to use regular non-atomic flags as they are immutable while the GC is
running.

Change-Id: I0a9f8ecb2eb2aafaf17e77626ae27604abd1b618
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3107230Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76471}