Auto merge of rust-lang#123356 - joboet:set_current_size, r=<try>

Reduce code size of `thread::set_current`

rust-lang#123265 introduced a rather large binary size regression, because it added an `unwrap()` call on a `Result<(), Thread>`, which in turn pulled its rather heavy `Debug` implementation. This PR fixes this by readding the `rtassert!` that was removed.

library/std/src/thread/mod.rs

@@ -684,9 +684,12 @@ thread_local! {
 
 /// Sets the thread handle for the current thread.
 ///
-/// Panics if the handle has been set already or when called from a TLS destructor.
+/// Aborts if the handle has been set already to reduce code size.
 pub(crate) fn set_current(thread: Thread) {
-    CURRENT.with(|current| current.set(thread).unwrap());
+    CURRENT.with(|current| {
+        rtassert!(current.get().is_none());
+        current.set(thread).unwrap();
+    });
 }
 
 /// Gets a handle to the thread that invokes it.