Assertion failure causes wrong exit status

``` rust
fn main() {
  os::args();
  assert!(false);
}
```

Causes the program to return with exit status 0. Thankfully removing `os::args()` makes it return with nonzero, but it appears that at any point after the invocation of `os::args()` an assertion failure will cause an exit status of 0.

I ran this against a compiler built at e51ec26dd7d915cfdfabcf5b6e32c001e30bd244
