Now that the cage is shared, we can remove the thread-local base and
instead use a simple global.

Bug: chromium:1325007
Change-Id: I05019de83868f823c66003740e277578f2a1d439
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3688051
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81037}