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
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.
MFC after: 1 week