MFC r345829, r345831

r345829:
powerpc: Apply r178139 from sparc64 to powerpc's fpu_sqrt

This fix was committed less than 2 months after the code was forked into the
powerpc kernel. Though powerpc doesn't use quad-precision floating point,
or need it for emulation, the changes do look like correctness fixes
overall.

This was found while trying to get fsqrt emulation working on e5500, which
does have a real FPU, but lacks the fsqrt instruction. This is not the
complete fix, the rest is to be committed separately.

r345831:
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.