arm64/disassem.c: add extended register instruction definitions

Add disassembly support for the following extended register
instructions: add, adds, sub, subs, cmp, cmn.

Reviewed by: mhorne
MFC after: 1 week
Pull Request: https://reviews.freebsd.org/D40967

(cherry picked from commit 4a07c778632bddb86a82f3e1fe144d889dae69c5)