Add test.

tests/ui/thread-local/spawn-hook-atexit.rs

@@ -0,0 +1,22 @@
+// Regression test for https://github.com/rust-lang/rust/issues/138696
+//@ run-pass
+
+#![feature(rustc_private)]
+
+extern crate libc;
+
+fn main() {
+    std::thread::spawn(|| {
+        unsafe { libc::atexit(spawn_in_atexit) };
+    })
+    .join()
+    .unwrap();
+}
+
+extern "C" fn spawn_in_atexit() {
+    std::thread::spawn(|| {
+        println!("Thread spawned in atexit");
+    })
+    .join()
+    .unwrap();
+}