[AArch64] Codegen support for FEAT_PAuthLR

- Adds a new +pc option to -mbranch-protection that will enable
  the use of PC as a diversifier in PAC branch protection code.

- When +pauth-lr is enabled (-march=armv9.5a+pauth-lr) in combination
  with -mbranch-protection=pac-ret+pc, the new 9.5-a instructions
  (pacibsppc, retaasppc, etc) are used.

Documentation for the relevant instructions can be found here:
https://developer.arm.com/documentation/ddi0602/2023-09/Base-Instructions/



Co-authored-by: Lucas Prates <lucas.prates@arm.com>