[gfortran,ping] support for large kinds in front-end and library

[Paul Brook]

> > On systems that incompletely support extended floating-points, we can
> > support them incompletely. We're not trying to write a complete OS :)
>
> This is a quality of implementation issue.  I can live with your
> suggestion below provided that I'm given carte blanche to implement
> the slower but higher quality MPFR versions.  Frankly, if I have

This implies requiring MPFR as a target library. This is not acceptable unless 
it is first integrated into the gcc build system. Requiring a host mprf is 
already contentious and undesirable. Requiring it for the target is even less 
desirable.

> > #ifndef HAVE_ATAN2L
> > long double atan2l (long double x)
> > { return (long double) atan2 ((double) x); }
> > #endif

Paul