Out-of-memory causes SIGILL

This snippet: ([Playpen link](http://is.gd/KI4QhD))

```
#![allow(unstable)]

extern crate test;

fn main() {
    let a: Vec<u8> = Vec::with_capacity(::std::usize::MAX);
    ::test::black_box(a);
}
```

Causes the program to close with a SIGILL (Illegal instruction) error. I would have expected something like a SIGSEGV instead.
