[AArch64][libunwind] Unwinding support for return address signing

- Follow up to revision r342895
- gcc would not build libunwind with the earlier patch as the autia1716
  instruction wasn't allowed to be assembled for pre armv8.3a targets
- The autia1716 instruction lives in the hint space encodings so is a valid
  instruction for all armv8a targets
- To work around this I have swapped out the autia1716 instruction for the hint
  instruction

Differential Revision: https://reviews.llvm.org/D55700

llvm-svn: 349140