Add safepoint mechanism to stop concurrent threads and bring them to a
safepoint. Threads are stopped before the safepoint and after e.g. the
GC resumed again. Each thread needs to be stopped in a safepoint, such
that all roots can be iterated safely.

Running threads need to be cooperative and are required to perform
regular safepoint polls.

The last version of this CL was reverted because safepoint_requested_
wasn't initialized (see https://crrev.com/c/2105634).

Bug: v8:10315
Change-Id: I6ef244c0fb31c178589b5e3d1c62687a8dd65768
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2105635Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66732}