depend-cleanup: Add a regex for files that moved

When a file is moved or copied to a new directory the depend-cleanup
script may incorrectly remove the .depend file as the generated regex
will pass. Fix this by passing in a new regex for the arm64 files that
have the same name as a generic implementation we were previously
using.

While here add memchr.S. It has moved from a generated file to one in
the source tree with the same name.

Reviewed by: emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D48519