Choose the page size based on V8_HOST_ARCH_ARM64 (i.e. we're building
an arm64 binary) instead of V8_TARGET_ARCH_ARM64 (i.e. V8's compilers
are emitting arm64 instructions, which is the case in simulator builds
as well).
Drive-by:
- use V8_TARGET_OS_MACOSX instead of __APPLE__
- drop implementation difference between AllocatePageSize and
  CommitPageSize on POSIX (they must return the same value anyway)

This continues and obsoletes the work at
https://chromium-review.googlesource.com/c/v8/v8/+/2314102 .

Bug: chromium:1107945, chromium:1128932
Change-Id: Iaaa509dd496ff581ddda4d957bc3d35d806cf81e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2421817
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70047}