Make current_thread_unique_ptr work during thread destruction.

Otherwise we can't use println!() within atexit handlers etc.

library/std/src/sys_common/thread_info.rs

@@ -34,7 +34,9 @@ fn with<R, F>(f: F) -> Option<R>
 ///
 /// This can be used as a non-null usize-sized ID.
 pub fn current_thread_unique_ptr() -> usize {
-    THREAD_INFO.with(|info| <*const _>::addr(info))
+    // Use a non-drop type to make sure it's still available during thread destruction.
+    thread_local! { static X: u8 = 0 }
+    X.with(|x| <*const _>::addr(x))
 }
 
 pub fn current_thread() -> Option<Thread> {