Maglev write barrier needs to use different page flag mask to also
call into the slow path of the barrier for old-to-shared pointers.

Bug: v8:11708
Change-Id: I9ff5ead0517e9682672452f9868b16fa0dd94be2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3834256Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Auto-Submit: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82549}