Change ThreadState representation from a fixed set of values to
either Parked or Running with two additional flags (or bits) that
are used when either a collection or a safepoint requested. Setting
either of these flags forces Park(), Unpark() and Safepoint() into
their slow path.

Currently we use the CollectionRequested flag on the main thread,
while SafepointRequested is used on background threads.

In case the slow path sees the CollectionRequested flag, it will
perform a GC. When encountering the SafepointRequested flag, the
background thread will participate in the safepoint protocol and
park itself for the duration of the safepoint operation.

This CL is a prerequisite for supporting safepoints across multiple
isolates. When safepointing multiple isolates, the main thread will
use both the CollectionRequested and SafepointRequested flag. This
isn't possible with the current system.

Design Doc: https://docs.google.com/document/d/1y6C9zAACEr0sBYMIYk3YpXosnkF3Ak4CEuWJu1-3zXs/edit?usp=sharing

Bug: v8:11708
Change-Id: I16b88740182d9c13bce54be163b334761529a5f0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3211894Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77349}