... by adding atomic (relaxed) accessor's for a map's
constructor_or_backpointer field, and using them in the two functions.

Bug: chromium:1250216, v8:7790
Change-Id: I3416799cca73792ff5f8963685274ad9afdc6229
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3162129Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76876}