Fix Miri build

Author: surban

library/std/src/sys/thread_local/key/unix.rs

@@ -29,5 +29,10 @@ pub unsafe fn destroy(key: Key) {
 
 #[inline]
 pub unsafe fn at_process_exit(cb: unsafe extern "C" fn()) {
+    // Miri does not support atexit.
+    #[cfg(not(miri))]
     assert_eq!(unsafe { libc::atexit(mem::transmute(cb)) }, 0);
+
+    #[cfg(miri)]
+    let _ = cb;
 }