std: Run at_exit cleanup on process::exit

This adds a call to `rt::cleanup` on `process::exit` to make sure we clean up
after ourselves on the way out from Rust.

Closes #28065

src/libstd/process.rs

@@ -582,6 +582,7 @@ impl Child {
 /// to run.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn exit(code: i32) -> ! {
+    ::rt::cleanup();
     ::sys::os::exit(code)
 }