TSAN complains about missing synchronization on access to the page flags
because it does not support and recognize the memory fence we emit after
page initialization.

This adds a TSAN only acquire load to the code accesses page flags
similar to the existing load in MarkObject.

Bug: v8:9842
Change-Id: I34dac308ac1cce1d74a4a1bad95a482abc071595
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1856008Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64247}