Replace 'virtual' by 'override' when overriding methods.
This uncovered one method which was unnecessarily virtual:
{RegExpMacroAssemblerARM64::CheckCharacters}.

R=jgruber@chromium.org

Bug: v8:12244
Change-Id: Ia4480b7b234d3d40cc5821c38ef83f74f8421b6b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3204966Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77252}