arm64: Add PAC instruction macros

To support Pointer Authentication (PAC) in assembly files add a pair of
macros that sign the link register. When used before storing to the
stack it will allow hardware to detect if it has changed before using
it in the return instruction.

Reviewed by: markj, emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42226