Add support for safepointing multiple isolates as described in the
design doc (link is below). A safepoint across multiple isolates is
considered a global safepoint to distinguish it from regular safepoints.

The basic idea behind the implementation is that we reach a
safepoint for each client. What's new is that now also main threads
need to participate in the safepointing protocol and need to give up
control in time. The slow paths of Park(), Unpark() and Safepoint() on
the main thread need to be adjusted for this reason as well.

This CL introduces GlobalSafepoint and GlobalSafepointScope to mirror
IsolateSafepoint and IsolateSafepointScope.

This CL adds the type IgnoreLocalGCRequests, it is used to prevent
Park() and Unpark() from honoring the request from background threads
to perform a local GC. This is used heap-internally to not have GCs
(or even nested GCs) in certain locations. E.g. when initiating a
safepoint to perform a GC we don't want a "recursive" GC to occur.

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

Bug: v8:11708
Change-Id: I5aca8f5f24873279271a53be3bb093fc92a1a1eb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3009224
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77812}