re-enable AMD Topology extension on certain models if disabled by BIOS

Some BIOSes disable AMD Topology extension on AMD Family 15h notebook
processors. We re-enable the extension, so that we can properly discover
core and cache topology. Linux seems to do the same.

Reported by: Johannes Dieterich <dieterich.joh@gmail.com>
Reviewed by: jhb, kib
Tested by: Johannes Dieterich <dieterich.joh@gmail.com>

		(earlier version)

MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D5883