[AArch64] Unwinding support for return address signing

- When return address signing is enabled, the LR may be signed on function entry
- When an exception is thrown the return address is inspected used to unwind the call stack
- Before this happens, the return address must be correctly authenticated to avoid causing an abort by dereferencing the signed pointer

Differential Revision: https://reviews.llvm.org/D51432

llvm-svn: 342895