rp2/mpthreadport: Ensure core1 doesn't hold gc lock in deinit.

Prior to this commit the following code would lock up the device when
Ctrl-D is entered at the REPL:

    import gc, _thread

    def collect_thread():
        while True:
            gc.collect()

    _thread.start_new_thread(collect_thread, [])

Fixes part of #8494.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

ports/rp2/mpthreadport.c

@@ -47,8 +47,13 @@ void mp_thread_init(void) {
 }
 
 void mp_thread_deinit(void) {
+    assert(get_core_num() == 0);
+    // Must ensure that core1 is not currently holding the GC lock, otherwise
+    // it will be terminated while holding the lock.
+    mp_thread_mutex_lock(&MP_STATE_MEM(gc_mutex), 1);
     multicore_reset_core1();
     core1_entry = NULL;
+    mp_thread_mutex_unlock(&MP_STATE_MEM(gc_mutex));
 }
 
 void mp_thread_gc_others(void) {