[patch,fortran] Fix PR19310 unnecessary error for overflowing results

[Andrew Pinski]

> > I don't buy that argument. The same is true for several of the other -f 
> > options (eg. -fbackslash) and we accept both forms of those.
> > 
> 
> Well, your counter argument isn't too compelling.
> IMHO, the handling of the other options is then broken.

And more to the point, it would be nice to do -fno-xxx -fxxx so
that you could enable it if you add to FFLAGS.

-- Pinski