msvcrt: Fix bugs in math.c

[Piotr Caban]

On 03/22/15 17:54, Zheng Chen wrote:
> Just came to me that maybe we should include isnan() in signbit()
> instead of using isnan() in _copysign(), seems like that's how Windows
> does it.
As far as I can see on MSDN signbit is only available in VS2012 and 
VS2013, I don't see any exports related to it in msvcr*/msvcp* dlls, so 
I guess it's inlined.

We use host system implementation of the function to implement some 
other functions in wine. That's why you can't change signbit implementation.

Thanks,
Piotr