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.

Bug: v8:10315
Change-Id: I47f07e7d2ef5bc5adbba6b9e8e79a1f0f45b97ad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2102578
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66727}