This change added Windows ARM64 ABI support, major things are:
1. Excluding x18 register from any usage because it is reserved as
   platform register. Preserve alignment after the change.
2. Fix the assumption of LP64 in arm64 backend. Windows ARM64 is
   still LLP64.
3. Stack guard page probe for large allocation on stack.

Reference:
Windows ARM64 ABI:
https://docs.microsoft.com/en-us/cpp/build/arm64-windows-abi-conventions?view=vs-2017

Bug: chromium:893460
Change-Id: I325884ac8dab719154a0047141e18a9fcb8dff7e
Reviewed-on: https://chromium-review.googlesource.com/c/1285129
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56881}