hwpmc: Always set pmc_cpuid to something

pmc_cpuid was uninitialized for most AMD processor families. We can still
populate this string for unimplemented families.

Also added a CPUID_TO_STEPPING macro and converted existing code to use it.

Reviewed by: mav
MFC after: 2 weeks
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D25673