arm64: clear debug register state on fork

Following the analogous change for amd64 and i386 in 8223717ce62c,
ensure that new processes start with these registers inactive.

PR: 254661
Reported by: Michał Górny
Reviewed by: kib, emaste
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 1fd001db9c330f133708f3c04c8852f8b07cfed9)