Lock stdout for the whole duration of print

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.

Author: nagisa

src/libstd/io/stdio.rs

@@ -398,7 +398,9 @@ pub fn _print(args: fmt::Arguments) {
                 return w.write_fmt(args);
             }
         }
-        stdout().write_fmt(args)
+        let stdout = stdout();
+        let mut lock = stdout.lock();
+        lock.write_fmt(args)
     });
     if let Err(e) = result {
         panic!("failed printing to stdout: {}", e);