fix-up for configuration of AMD Family 10h processors borrowed from Linux

http://lxr.free-electrons.com/source/arch/x86/kernel/cpu/amd.c#L643
BIOS may configure Family 10h processors to convert WC+ cache type
to CD. That can hurt performance of guest VMs using nested paging.

Reviewed by: kib
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D6059