arm64/disassem.c: Add support str/strb/strh instructions

Added disassembly support for each type of str/strb/strh instruction
encoding.

Reviewed by: mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39336

(cherry picked from commit 7edb7adf8c8b0615c6d845ca789d4f527f8b18a3)