Rename arm64 macros in preperation for a script to generate them.

I have a script to generate most of the ID_AA64* macros from the Arm
XML source [1]. In preperation for using this we need to clean up the
macros to be in line with what the script will generate. This is the
first step, rename the macros to follow the names in said XML.

[1] https://developer.arm.com/architectures/cpu-architecture/a-profile/exploration-tools

MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D20976