[PATCH, fortran] Interoperability with C int128_t types

[Sa Liu]

Thanks a lot for the review. I will modify the patch and run the 
testsuites again.

>   if (gfc_notify_std (std_for_isocbinding_symbol (s),
>                             "Extension: '%s' symbol in ISO_C_BINDING
>  intrinsic module at %C",
>                             name) == FAILURE)
>     return;

I don't think we should issue an error here, when generating the 
isocbinding symbols. Even if no extension types are used in program, the 
compiler will go through the type definitions and check their standards. 
In stead, we could just ignore the extension types and don't generate 
symbols for them if -std=f2003:

  if (gfc_notification_std (std_for_isocbinding_symbol (s)) == FAILURE)
    return;

Sa