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

[pnkfelix]

Test case playpen

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

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

[pnkfelix]

cc @eddyb @nikomatsakis

[petrochenkov]

One more interesting example:

extern "rust-call" {
    // Foreign items
}

I'll fix this.

[nikomatsakis]

D'oh. Does seem like we ought to feature-gate this. I don't envision a lot of trouble.

[nikomatsakis]

Discussed ABIs in context of the PR, denominating.