[patch, fortran] Fix PR 87689, wrong decls / ABI violation on POWER

[Thomas Koenig]

Bob,

> Some of us still use varargs interfaces (in my case, Fortran calling C 
> stdarg subroutines).

The problem for us is that that sometimes using varargs made standard-
conforming Fortran code like, in file a.f

       subroutine foo(a)
       print *,a
       end

and in file main.f

       programme main
       call foo(1.0)
       end

depend ABI details: The call to foo used to be called using
the varargs convention, and the subroutine foo was compiled
as a non-varargs function.

This "worked" until PR 87689 showed that this breaks
standard-conforming Fortran code on a primary gcc platform.

I don't know if that makes a difference for the platform you work
on.  For the System V AMD64 ABI, I suspect it actually might not
matter (at least from glancing at the corresponding Wikipedia
article), but I am _not_ an expert in this field, so please take this
with a chunk of rock salt of appropriate size.

So, we cannot really keep this as a feature (note that varargs
are also not C interoperable).

Regards

	Thomas