powerpc: Allow emulating optional FPU instructions on CPUs with an FPU

The e5500 has an FPU, but lacks the optional fsqrt instruction. This
instruction gets emulated in the kernel, but the emulation uses stale data,
from the last switch out, and does not return the result of the operation
immediately. Fix both of these conditions by saving and restoring the FPRs
around the emulation point.

MFC after: 1 week
MFC with: r345829