powerpc: Rework IPI message processing

[jhibbits]

Summary

There's no need to use a while loop in the IPI handler, the message list
is cached once and processed. Instead, since the existing code calls
ffs(), sort the handlers, and use a simple 'if' sequence.

Test Plan

Boot a SMP PowerPC system

[nwhitehorn]

Looks good to me, assuming it works anyway.

sys/powerpc/powerpc/mp_machdep.c

323

There's no switch anymore here.