For Xeon 7500 and 48XX (Nehalem EX and Westmere EX) variants of the
Core i7 and Westmere processors, the uncore PMC subsystem is
completely different from the uncore PMC on smaller versions of CPUs.
Disable existing uncore hwpmc code for EX, otherwise non-existing MSRs
are accessed.

The cores PMCs seems to be identical for non-EX and EX, according to
the SDM.

Reviewed by:	davide, fabient
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks