The pointer compression cage is the virtual memory reservation
that all compressed pointers fall within. This CL splits pointer
compression into two modes: a per-Isolate cage and a shared cage
among multiple Isolates.

When multiple Isolates are sharing a cage, they can decompress
each others' pointers and share the same virtual memory range.

Bug: v8:11460
Change-Id: I7b89b7413b8e7ca6b8b6faafd083dc387542a8b4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2783674Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73916}