Enable compaction of objects in the map space during a full GC. So far
pages in the map space were never chosen as evacuation candidates. We
might be able to improve memory usage a bit by also compacting map
space. Luckily for us the marking barrier was already emitted when
updating an object's map word.

This CL adds a new flag FLAG_compact_map_space to easily turn off this
feature again. For now we keep this flag (and with that map space
compaction) disabled by default. So GC behavior does not change with
this CL.

Bug: v8:12578
Change-Id: I99c0cd826bd824af5383fb3ce64796693a59d1ff
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3404775Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78844}