Mistakenly had its set as non-atomic when it was modifying maps the
concurrent compiler could see. Since this accessor is set after map
initialization, but it is not necessary for synchronization we can set
it as relaxed write.

Bug: v8:7790, v8:11668
Change-Id: I605935e96b3da47ed6abfb6676bf14456feeeddd
Fixed: v8:11668
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2839548Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74066}