Enable some pointer authentication instructions for aarch64 v8a targets

- Some of the v8.3 pointer authentication instruction inhabit the Hint space
- These instructions can be assembled to hint instructions which act as NOP instructions prior to v8.3
- This patch permits using the hint instructions for all v8a targets
- Also, correct the RETA{A,B} instructions to match the instruction attributes of RET (set isTerminator and isBarrier)

Differential Revision: https://reviews.llvm.org/D49786

llvm-svn: 338029