Previously, guard regions were created by allocating pages with
PROT_NONE and relying on an allocation hint. This could fail however,
for example on Fuchsia (where it would allocate a VMO to back the guard
region) and possibly on Windows (where a placeholder mapping was
replaced by a "real" mapping).

Introducing an explicit VirtualAddressSpace::AllocateGuardRegion routine
now makes this operation more efficient and effectively guarantees that
it cannot fail if used correctly: in a regular subspace, there is no
need to allocate anything when creating guard regions since the address
space reservation backing the subspace is guaranteed to be inaccessible
when no pages are allocated in it.

Bug: chromium:1218005
Change-Id: I6945f17616b6b8dad47241af96d4cb1f660e8858
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/+/3366237Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78480}