`rust-call` call ABI is not feature gated in all points of declaration

Test case [playpen](https://is.gd/b3Yq4i)

``` rust
fn foo() { println!("foo"); }

fn main() {
    let f: extern "rust-call" fn((), ());
    f = unsafe { std::mem::transmute(foo as fn()) };
    f((), ())
}
```
