Syntax-phase crates are always linked against

It appears that this is was caused by #13384.

``` rust
#![feature(phase)]
#[phase(syntax)]
extern crate fourcc;

fn main() { fourcc!("1234"); }
```

```
~ ❯ rustc -Z lto test.rs
error: could not find rlib for: `fourcc`
```

cc @alexcrichton 
