Add support for compare and swap in memory instructions.

Part of Armv8.1 Large System Extensions. Includes:
 - CAS, CASA, CASL, CASAL - Compare and swap word or doubleword in memory.
 - CASB, CASAB, CASLB, CASALB - Compare and swap byte in memory.
 - CASH, CASAH, CASLH, CASALH - Compare and swap halfword in memory.
 - CASP, CASPA, CASPL, CASPAL - Compare and swap pair of words or doublewords in memory.

Change-Id: I1b55b4f53a987c455d1cbc96210856ebeb9f63bf