Implement reentrant mutexes and make stdio use them

write_fmt calls write for each formatted field. The default implementation of write_fmt is used,
which will call write on not-yet-locked stdout (and write locking after), therefore making print!
in multithreaded environment still interleave contents of two separate prints.

This patch implements reentrant mutexes, changes stdio handles to use these mutexes and overrides
write_fmt to lock the stdio handle for the whole duration of the call.

src/test/run-pass/atomic-print.rs

@@ -0,0 +1,48 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::{env, fmt, process, sync, thread};
+
+struct SlowFmt(u32);
+impl fmt::Debug for SlowFmt {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        thread::sleep_ms(3);
+        self.0.fmt(f)
+    }
+}
+
+fn do_print(x: u32) {
+    let x = SlowFmt(x);
+    println!("{:?}{:?}{:?}{:?}{:?}", x, x, x, x, x);
+}
+
+fn main(){
+    if env::args().count() == 2 {
+        let barrier = sync::Arc::new(sync::Barrier::new(2));
+        let tbarrier = barrier.clone();
+        let t = thread::scoped(||{
+            tbarrier.wait();
+            do_print(1);
+        });
+        barrier.wait();
+        do_print(2);
+        t.join();
+    } else {
+        let this = env::args().next().unwrap();
+        let output = process::Command::new(this).arg("-").output().unwrap();
+        for line in String::from_utf8(output.stdout).unwrap().lines() {
+            match line.chars().next().unwrap() {
+                '1' => assert_eq!(line, "11111"),
+                '2' => assert_eq!(line, "22222"),
+                _   => panic!("Unexpected character")
+            }
+        }
+    }
+}