Fix some corner cases:

Fix some corner cases:
- fma(x, y, z) returns z, not NaN, if z is infinite, x and y are finite,
  x*y overflows, and x*y and z have opposite signs.
- fma(x, y, z) doesn't generate an overflow, underflow, or inexact exception
  if z is NaN or infinite, as per IEEE 754R.
- If the rounding mode is set to FE_DOWNWARD, fma(1.0, 0.0, -0.0) is -0.0,
  not +0.0.