On 64bit we guarantee that object alignment and sizes are multiple of
the default shadow memory granularity (8 bytes). The CL also introduces
CHECKs that the assumption holds.

Having kObjectAlignment be multiple of this granularity allows us to
check poisoness of each byte of an object. On 32bit we can not do that,
since the object alignment requirement is 4 bytes.

Bug: chromium:1241514
Change-Id: Ib19667724adaa7bc791ffa054eea618c365d65cb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3118552
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76499}