rustc_driver: Use the target tools PATH

When cross-compiling the PATH for the target should be favored (instead of the
host) because if we're going to run some custom tools then we likely want to
run the custom tools from the target, not the host (e.g. if GNU windows is
compiling to MSVC windows).

Author: alexcrichton

src/librustc_driver/driver.rs

@@ -761,7 +761,8 @@ pub fn phase_6_link_output(sess: &Session,
                            trans: &trans::CrateTranslation,
                            outputs: &OutputFilenames) {
     let old_path = env::var_os("PATH").unwrap_or(OsString::new());
-    let mut new_path = sess.host_filesearch(PathKind::All).get_tools_search_paths();
+    let mut new_path = sess.target_filesearch(PathKind::All)
+                           .get_tools_search_paths();
     new_path.extend(env::split_paths(&old_path));
     env::set_var("PATH", &env::join_paths(new_path.iter()).unwrap());