sparc64: Implement a real set_perf_counter_pending().

When the perf counter subsystem needs to reschedule work out
from an NMI, it invokes set_perf_counter_pending().

This triggers a non-NMI irq which should invoke
perf_counter_do_pending().

Currently this won't trigger because sparc64 won't trigger
the perf counter subsystem from NMIs, but when the HW counter
support is added it will.

Signed-off-by: David S. Miller <davem@davemloft.net>