PCScan: Implement lazy safepoints

Safepoints strengthen PCScan security by blocking mutators from doing
potentially malicious work. Laziness allows to avoid eager
stop-the-world and block only regular executed code of the attacker
(malloc/free). While blocking, mutators have a chance to help out
PCScan.

This doesn't implements raceful wait-free scanning for simplicity
reasons, but this can be addressed in the future, if blocking becomes an
issue.

The CL introduces safepoints only on the malloc path.

Followups:
1) Reduce snapshotting.

Bug: 11297512
Change-Id: Id990fd96ab118bba561726fa427d33d4962c72c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2765642


Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#864965}