arm64: Add MRS_REG_ALT_NAME ID register macros

These can be used even when the compiler is too old for the register
to be included.

Reviewed by: Zachary Leaf <zachary.leaf@arm.com>
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45176

(cherry picked from commit d6d860c7ff5c4cbe9475d98000407d6f0ea84b47)