Add note about using cells in the locks on the 'unsupported' platform.

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

@@ -1,6 +1,7 @@
 use crate::cell::Cell;
 
 pub struct Mutex {
+    // This platform has no threads, so we can use a Cell here.
     locked: Cell<bool>,
 }
 

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

@@ -1,6 +1,7 @@
 use crate::cell::Cell;
 
 pub struct RWLock {
+    // This platform has no threads, so we can use a Cell here.
     mode: Cell<isize>,
 }