It could happen (e.g. with --stress-background-compile) that the main
thread blocks for a background thread but the background thread requests
a GC from the main thread. This would result in a deadlock. Avoid this
by parking the main thread for potentially blocking operations and allow
allocations while the main thread is parked.

This CL introduces new states for the main thread: CollectionRequested
and ParkedCollectionRequested. These states will force Safepoint(),
Park() and Unpark() on the main thread into slow paths. The slow path
can then perform a GC on the main thread - right before parking or after
the main thread got unparked.

Bug: v8:10315
Change-Id: If7ef31622d27320613139a0b7f79086fe3200f99
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2731528Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73707}