Refactor strtod parsing of NaN payloads

[Florian Weimer]

On 11/21/2015 01:10 AM, Joseph Myers wrote:

> 	* include/stdlib.h (__strtof_nan): Declare and use
> 	libc_hidden_proto.
> 	(__strtod_nan): Likewise.
> 	(__strtold_nan): Likewise.
> 	(__wcstof_nan): Likewise.
> 	(__wcstod_nan): Likewise.
> 	(__wcstold_nan): Likewise.

This could be marked internal_function, I think (both the declaration
and definition).

Looks good to me.

Thanks,
Florian