Replace a manual fetch/or/store sequence with fetch_or.

Bug: chromium:1056170
Change-Id: Ib18d26686aa6e822f91b5b6662dd0bdcfafe5092
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2531788
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71129}