Prefinalizers have long been forbidden to allocate.
This restriction often proved problematic and has caused several
issues in the past.

This CL adds support for allowing allocations in prefinalizers.
At the start of prefinalizer invocations we clear the linear
allocation buffers, such that all allocations go through the slow
path for allocation. The slow path checks whether prefinalizers
are currently being invoked and marks the newly allocated object
if they are (i.e. black allocation during prefinalizers).

The new behavior is disabled by default and can be enabled by
setting the cppgc_allow_allocations_in_prefinalizers gn arg to true.

Bug: chromium:1056170
Change-Id: Ib86e780dcff88fa7b0f762ac2ab83c42393d33af
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3097877
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76606}