[libstdc++] Expose Airy functions.

[Ed Smith-Rowland]

On 11/17/2017 10:00 AM, Jonathan Wakely wrote:
> Also, this reminds me to ask about the __isnan helper in
> <tr1/special_function_util.h>, can we get rid of it? It looks to me
> like we always have a usable std::isnan, no?

Let's get rid of __isnan.  I've actually had trouble with it.

Ed