The arm64 ABI defines x18 as a platform register, and as such
platforms may reserve it for their own purposes.

This CL unconditionally removes x18 from the allocatable register list
(previously it was only excluded from arm64 Windows). If, for some
reason, we want to keep x18 allocatable on some platforms, we can
explicitly enable it for specific platforms in the future.

Bug: v8:8940,v8:9140
Change-Id: I28c4f6aad714e21a0a54bab6041c13a1b28fd467
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1564194Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60870}