[cleanup] Change base::CPU to use kCamelCase

ARM is often defined as a macro so this changes it to kArm and fixes
other cases in the same file.

Bug: v8:11384
Change-Id: Iab0149be03b3b0139e3335b91a25cb4bbb2f56e3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2808939


Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73826}