powerpc/perf: move perf irq/nmi handling details into traps.c

This is required in order to allow more significant differences between
NMI type interrupt handlers and regular asynchronous handlers.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210130130852.2952424-20-npiggin@gmail.com