{f32,f64,float}::tests::test_abs_sub unit tests in libstd fail on Windows

`f32::tests::test_abs_sub`, `f64::tests::test_abs_sub and float::tests::test_abs_sub` in libstd fail this assertion: `assert!(NaN.abs_sub(&-1f32).is_NaN());  (the result is 0 instead of NaN)`

This is because mingw's implementation of underlying function `fdim()` does not take NaNs into account:

``` c
double fdim (double x, double y)
{
  return  (isgreater(x, y) ? (x - y) : 0.0);
}
```
