Rearrange SGX split module files

In rust-lang#75979 several inlined modules were split out into multiple files.
This PR keeps the multiple files but moves a few things around to
organize things in a coherent way.

library/std/src/sys/sgx/waitqueue.rs → library/std/src/sys/sgx/waitqueue/mod.rs

@@ -13,13 +13,8 @@
 #[cfg(test)]
 mod tests;
 
-/// A doubly-linked list where callers are in charge of memory allocation
-/// of the nodes in the list.
-mod unsafe_list;
-
-/// Trivial spinlock-based implementation of `sync::Mutex`.
-// FIXME: Perhaps use Intel TSX to avoid locking?
 mod spin_mutex;
+mod unsafe_list;
 
 use crate::num::NonZeroUsize;
 use crate::ops::{Deref, DerefMut};

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

@@ -1,3 +1,6 @@
+//! Trivial spinlock-based implementation of `sync::Mutex`.
+// FIXME: Perhaps use Intel TSX to avoid locking?
+
 #[cfg(test)]
 mod tests;
 

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

@@ -1,3 +1,6 @@
+//! A doubly-linked list where callers are in charge of memory allocation
+//! of the nodes in the list.
+
 #[cfg(test)]
 mod tests;