arm64: Use the pointer auth register defines

When building with gcc it complains the pointer authentication
registers aren't valid with the architecture level we are targeting.
Fix this by using the alternative spelling of these registers accesses
through MRS_REG_ALT_NAME.

Reviewed by: jhb
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45263

(cherry picked from commit 73c200447361a649e00cdb1693a0ae7f3291914e)