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
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29630