It is always a good idea. In one case, attempting to allocate N+1 PMCs
from a class with N hardware counters would incorrectly attempt to
allocate from the next class in the list. Without this validation, this
can lead to all kinds of unexpected behaviour.
Since powerpc_allocate_pmc() is used by both the mpc7xxx and ppc970
classes, add a new global to track which is active (it will never be