Only store the arm64 ID registers in the cpu_desc

There is no need to store a pointer to the CPU implementer and part
strings. Switch to load them directly into the sbuf used to print them
on boot.

While here print the machine ID register when we fail to determine the
implementer or part we are booting on.

Reviewed by: markj, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31346