This is a step towards making JSObjectRef non-serialized.

Change JSObjectRef::RawFastPropertyAt to use a direct load with
relaxed semantics. Special handling of `uninitialized` sentinel values
is moved to the only use-site.

A new lock `boilerplate_migration_access` protects against concurrent
boilerplate migrations while we are iterating over properties.

Bug: v8:7790
Change-Id: Ic9de54ca16c1f3364d497a77058cfa33d48dd4a4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2928184
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75033}