box mutex to get a movable mutex

the commit avoid an alignement issue in Mutex implementation

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

@@ -155,7 +155,7 @@ pub struct Mutex {
     inner: Spinlock<MutexInner>,
 }
 
-pub type MovableMutex = Mutex;
+pub type MovableMutex = Box<Mutex>;
 
 unsafe impl Send for Mutex {}
 unsafe impl Sync for Mutex {}