libsyntax/librustc: Allow calling variadic foreign functions.
Add tests for variadic foreign functions.
auto merge of #10064 : luqmana/rust/vvv, r=nikomatsakis

Fixes #2057.

Example:
```Rust
#[no_std];

type c_char = u8;
type c_int = i32;
type size_t = uint;

extern {
    fn printf(format: *c_char, ...) -> c_int;
}

#[lang="fail_bounds_check"]
fn fail_bounds_check(_: *c_char, _: size_t, _: size_t, _: size_t) {}

#[start]
#[fixed_stack_segment]
fn main(_: int, _: **u8) -> int {
    unsafe {
        let msg = bytes!("Hello World!