[threads] Fix leaking threads: SGen Worker and Finalizer (#5284)

[threads] Fix leaking threads: SGen Worker and Finalizer (#5284)

* Fix leaking threads: SGen Worker and Finalizer
- https://bugzilla.xamarin.com/show_bug.cgi?id=58317
- stop SGen workers before the finalizer stops
- clean up SGen workers with the Finalizer
- remove `mono_thread_join (GUINT_TO_POINTER (gc_thread->tid));` as we detach the Finalizer with `mono_threads_join_threads ()` within `mono_gc_cleanup ()`

* [fixup!] Move counting variable into the loop initialiser and add a `(gpointer)` cast