powerpc: Set very low priority mode while waiting for AP unleash event

The POWER9 does not recognize 'or 27,27,27' as a thread priority NOP. On
earlier POWER architectures, this NOP would note to the processor to give up
resources if able, to improve performance of other threads.

All processors that support the thread priority NOPs recognize the
'or 31,31,31' NOP as very low priority, so use this to perform a similar
function, and not burn cycles on POWER9.