Instead of explicitely splitting the cage into two separate regions, we
now just create a single BoundedPageAllocator to manage the entire
address range of the cage, then allocate the first 4GB for the pointer
compression cage.

Bug: chromium:1218005
Change-Id: I02c53ca8b6dda9074ae6caccc74c32bd6271d4d2
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3162044Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76900}