This CL ports the atomic object start bitmap from blink.
Using the bitmap for mixin tracing is left as a followup.

Bug: chromium:1056170
Change-Id: I4998a0d9d76708a7bab0634e04354809dfc8c78f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2287504Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68778}