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

[vadimcn]

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:

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

[vadimcn]

Apparently this has been fixed in mingw-w64.