This CL introduces the --separate-gc-phases flag, which prevents that young and old generation GCs can happen at the same time. When incremental marking is in progress and a young generation GC triggers, marking is forced to finish and the full collection takes care of the young generation.

Bug: v8:12503
Change-Id: Ia3e4814f46bff0fdc404b0ac618dfd48fe7cf20c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3351973Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79799}