Hi all,

forth patch of the series adding support for PACBTI for Cortex-M.

This is to add the 'pac' instruction.

See:
- Armv8.1-M Pointer Authentication and Branch Target Identification Extension [1]
- Armv8-M Architecture Reference Manual [2]

The series was tested and does not introduce regressions.

Regards

  Andrea

[1] <https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/armv8-1-m-pointer-authentication-and-branch-target-identification-extension>
[2] <https://developer.arm.com/documentation/ddi0553/latest>