This low-level API implements virtual address space reservations on the
supported platforms. An AddressSpaceReservation supports similar
functionality as the global page management APIs in the OS class but
operates inside a continuous region of previously reserved virtual
address space. A reservation is backed by regular mmap mappings on
Posix, by placeholder mappings on Windows, and by VMARs on Fuchsia.

Bug: chromium:1218005
Change-Id: I99bc6bcbc26eb4aa3b54a31c671c9e06e92c471b
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3270540
Commit-Queue: Samuel Groß <saelo@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78130}