Add more arm64 special registers
These will be used by bhyve
Reviewed by: markj
Sponsored by: Arm Ltd
Sponsored by: Innovate UK
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40128
(cherry picked from commit 419f8fc7fbc8b860df92db58d13ac9c9aafa1c10)