pmc.amd(3): rename from pmc.k8(3)

This is the first step in recognizing that the K8 microarchitecture
represents a small and aged subset of AMD CPUs supported by this class.
Future changes will update the code and documentation details to better
reflect this.

Keep the old filename as an alias.

Reviewed by: jkoshy
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41279