Rollup merge of #87507 - jethrogb:jb/sgx-unmoveable-mutex, r=dtolnay

SGX mutex is *not* moveable

Reverts the erroneous change in #85029.

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

@@ -8,7 +8,8 @@ pub struct Mutex {
     inner: SpinMutex<WaitVariable<bool>>,
 }
 
-pub type MovableMutex = Mutex;
+// not movable: see UnsafeList implementation
+pub type MovableMutex = Box<Mutex>;
 
 // Implementation according to “Operating Systems: Three Easy Pieces”, chapter 28
 impl Mutex {

library/std/src/sys/sgx/waitqueue/unsafe_list.rs

@@ -23,6 +23,7 @@ impl<T> UnsafeListEntry<T> {
     }
 }
 
+// WARNING: self-referential struct!
 pub struct UnsafeList<T> {
     head_tail: NonNull<UnsafeListEntry<T>>,
     head_tail_entry: Option<UnsafeListEntry<T>>,