Avoid data race when enabling flag with concurrent thread that reads
that flag as well.

Bug: v8:10315
Change-Id: I9eecc48e21b1070e8db444021264eec2784f5102
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2567697
Auto-Submit: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71528}