Sync the arm64 special registers with the Armv8.5 XML

Add the missing macros and decode all the fields as described in the
Arm Architecture System Registers XML corresponding to Armv8.5.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30983

(cherry picked from commit a7b05eb16c9d84e1fd59864f5da67d23897ed91c)