1) add relaxed version of Code::main_cage_base accessors and use them
   from in those cases where they can be called from backround thread,
2) pass the main cage base value to IsCode() predicate to avoid
   accessing non-acomic Heap pointer value in page headers from
   background compilation thread.

Drive-by cleanup: use MarkingVerifier::cage_base() instead of
Code::main_cage_base().

Bug: v8:11880, v8:12611
Change-Id: I9fd28c1a3babb862d08fec09f6cfc369beaad231
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3494238Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79313}