[PowerPC64LE] Fix AP spinup on powernv.

OPAL unconditionally enters secondary CPUs with only HV and SF set.

I tried writing a secondary entry point instead, but OPAL rejected it
and I am unsure why, so I resorted to making the system reset interrupt
endian-flexible.

This means we take a slight performance hit on wakeup on LE, but it is
a good stopgap until we can figure out a reliable way to make OPAL enter
where we want it to.

It probably makes sense to have it around anyway, because I can imagine
scenarios where the cpu resets itself to BE and does a software reset.

Sponsored by: Tag1 Consulting, Inc.