work around bug when calling c-stack-cdecl fns from outside crate

work around bug when calling c-stack-cdecl fns from outside crate

In the main test, I call the wrapper instead of the native fn, as intended.
I also added an xfail-test that exercises the broken code path.  Will
file a bug.

Description of the broken code path:

The code path is that when we look up the external identifier we go through
trans_external_path() ->  type_of_ty_param_kinds_and_ty() ->
type_of_fn_from_ty() -> type_of_fn(), and type_of_fn() adds a lot of external
parameters. Problem is, I guess, that we don't pass the native ABI (or even the
fact that it's a native function!), just the types and kinds of the parameters.