Avoid stray sweeper tasks still accessing heap after teardown has been
initiated. This reverts https://crrev.com/c/2297466 and restores the
invariant that no GC is running during teardown.

Bug: v8:12025
Change-Id: I9974a63ef385783a9c3ec09725dce118f754b25f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3081615Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76219}