The FreeBSD by default "disables" hyper-threading cores, by not scheduling

The FreeBSD by default "disables" hyper-threading cores, by not scheduling
any threads to them. However, it still counts those cores as "active but
permanently idle" when calculating system-wide CPUs statistics. It is
incorrect, since it skews statistics quite a bit and creates real problems
for certain types of applications (monitoring applications for example),
by making them believe that the system does have enough idle CPU resources,
while in fact it does not.

Correct the problem by not calling performance counting routines on "disabled"
cores. The cleaner solution would be to just disable APIC timer interrupts on
those cores completely, but ENOTIME here and it is not clear if the
additional complexity really worth minor performance gain.

Reviewed by:	ssouhlal
Sponsored by:	Sippy Software, Inc.
MFC after:	2 weeks