Don't allow array types in extern "C" functions.

You cannot pass an array to or return an array from a C function. `f(char x[N])` is equivalent to `f(char *x)` for every N. On x86_64 linux, the following two extern "C" functions produce the same assembly:

``` rust
extern "C" {
    fn f(x: [i32, ..4]); // void f(int x[4])
    fn g(x: i64, y: i64);
}
```

This will segfault.
