Linking dylib to rlib doesn't emit error, although reversed one does

a.rs:

``` Rust
#[crate_type = "dylib"];
```

b.rs:

``` Rust
#[crate_type = "rlib"];
extern mod a;
```

Then `liba-ca3f740d-0.0.so` and `libb-920859ee-0.0.rlib` are built successfully.

The reversed case causes error as expected:

```
error: could not find dynamic library for: `a`
```
