SynchronizePageAccess is used to synchronize between page initialization
and reads from that page. It was not used for main thread reads because
it was assumed that all pages are initialized on the main thread. With
concurrent allocations, pages may be concurrently initialized, thus
requiring a fence for main threads reads as well.

Bug: v8:13041
Change-Id: I93e5162243ef5458579f239b131094d7171e8615
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3752804Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81630}