Now that the cage is shared, its metadata must be thread-safe.

Bug: chromium:1336529
Change-Id: I0650462d1faf171fc3325808ca45ebe044e91f45
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3707097
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81176}