Use `padreg` instead of x18 to maintain alignment in the CPURegList.

Also clean up some comments and tidy up RequiredStackSizeForCallerSaved
and PushCallerSaved.

Change-Id: I80a780e5649e69a1746c43f37c2d1d875120c7a0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1581609Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#60987}