This CL fixes isolate deserialization such that the first test for
concurrent allocation in the shared spaces already succeeds.

* Allows dereferences for the shared heap and not just for the shared
  isolate.
* Updates shared_heap_object_cache() for --shared-space.
* Sets IN_SHARED_HEAP flag on all shared space pages.

Bug: v8:13267
Change-Id: I912630da34f93e15d2ddef77a45a5e875bdceff0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3902523
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Jakob Linke <jgruber@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83320}