We were already performing these accessors in an atomic relaxed way for
concurrent marking. In no-cm builds I thought we could get away with
performing it non-atomically but we are seeing TSAN warnings.

Bug: v8:7790, v8:11945
Change-Id: I4f3b1be3e2ae726ac15777e6eb464979b3c0159c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3001179
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75568}