[AArch64] - return address signing

- Add a command line options -msign-return-address to enable return address
  signing
- Armv8.3a added instructions to sign the return address to help mitigate
  against ROP attacks
- This patch adds command line options to generate function attributes that
  signal to the back whether return address signing instructions should be
  added

Differential revision: https://reviews.llvm.org/D49793

llvm-svn: 340019