perfcounters: IRQ and NMI support on AMD CPUs

The below completes the K7+ performance counter support:

 - IRQ support
 - NMI support

KernelTop output works now as well.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Jaswinder Singh Rajput <jaswinder@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1236273633.5187.286.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>