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.