Fix large ulp error in pow without fma very near 1.0

The !HAVE_FAST_FMA code path split r = z/c - 1 into r = rhi + rlo such
that when z = 1-tiny and c = 1 then rlo and rhi could have much larger
magnitude than r which later caused large rounding errors.

So do a nearest rounding instead of truncation at the split.