No longer put mutexes on the 'unsupported' platform in a box.

These mutexes are just a bool (in a cell), so can be moved without
problems.

library/std/src/sys/unsupported/mutex.rs

@@ -4,7 +4,7 @@ pub struct Mutex {
     locked: UnsafeCell<bool>,
 }
 
-pub type MovableMutex = Box<Mutex>;
+pub type MovableMutex = Mutex;
 
 unsafe impl Send for Mutex {}
 unsafe impl Sync for Mutex {} // no threads on this platform