This is a bit hackish and should be made more generic (ie, support more than
two hard-coded performance counter+config register pairs) so it can be used
for mips74k and other chips.
All this does is process the initial interrupt event. It doesn't (yet) handle
callgraph events, so even if you route the exception/interrupt to this routine
and flip the bit on, it will hang and crash pmc unless you disable callgraph
support when you enable a sample based PMC.
