Limit workaround for errata E400 to appropriate AMD cpus.

From Linux sources and several datasheets I looked at, it seems that
the workaround is only needed on families 0xf and 0x10. For instance,
Ryzens do not implement the accessed MSR at all, it is documented as
reserved. Also, hypervisors should not allow guest to put CPU into
idle state, so activate workaround only when on bare hardware.

While there, style the code:

move MSR defines to specialreg.h
move identification to initcpu.c

Reported by: whu
Reviewed by: avg
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D26470