Fix data race between concurrent threads allocating (accessing gc_state_
that way) and the main thread starting tear down.

Bug: v8:10315
Change-Id: Icc24811e43268512c8d7fdaf92ecd3fc7b3ecd57
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2297390Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68853}