rustc: Make monitor public.

It's harder to run rustc correctly without it.

src/librustc/driver/mod.rs

@@ -418,7 +418,7 @@ pub fn list_metadata(sess: &Session, path: &Path,
 ///
 /// The diagnostic emitter yielded to the procedure should be used for reporting
 /// errors of the compiler.
-fn monitor(f: proc():Send) {
+pub fn monitor(f: proc():Send) {
     // FIXME: This is a hack for newsched since it doesn't support split stacks.
     // rustc needs a lot of stack! When optimizations are disabled, it needs
     // even *more* stack than usual as well.