[AArch64] Add ARMv8.2-A UAO PSTATE bit

ARMv8.2-A adds a new PSTATE bit, PSTATE.UAO, which allows the LDTR/STTR
instructions to behave the same as LDR/STR with respect to execute-only
pages at higher privilege levels. New variants of the MSR/MRS
instructions are added to allow reading and writing this bit. It is a
required part of ARMv8.2-A, so no additional subtarget features are
required.

Differential Revision: http://reviews.llvm.org/D15020

llvm-svn: 254157