ICE: `--linker=`

```
$ rustc.exe a.rs --linker=
task 'rustc' failed at 'Unhandled condition: io_error: io::IoError{kind: OtherIoError, desc: "Unknown system error", detail: None}', C:\home\stone\rust-vanilla\src\libstd\condition.rs:131
error: internal compiler error: unexpected failure
```

(reproduced both on win32 and linux)
#10754 blocks this.
