Sort the names of the arm64 debug registers

While here clean up the names for the naming convention of the other
registers in this file.

Reviewed by: kib, mhorne (earlier version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34060

(cherry picked from commit 664640ba6ccd0c14b607e9babd101dc85ac9c28a)