cppgc: Always keep ObjectStartBitmap in consistent state

Currently, OSB can not be safely accessed if sweeping is in progress.
This can, however, be easily lifted with atomic stores.

Having the consistent bitmap is needed for the generational barrier for
source objects (to retrieve the source object beginning).

Bug: chromium:1029379
Change-Id: I5fb8db579f881ddf240ce68ad51fa8264ee645dc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3545071


Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79606}