rp2/pendsv: Fix PendSV_Handler dispatch check when threading enabled.

If MICROPY_PY_THREAD is set, PendSV_Handler acquires a mutex and calls the
dispatch functions. If pendsv_schedule_dispatch is called by a higher
priority interrupt while the mutex is acquired by PendSV_Handler it's
possible for the dispatch to not be triggered.

Add a check for dispatch calls at the end of PendSV_Handler once the mutex
has been released.

Fixes issue micropython#18365.

Signed-off-by: Peter Harper <peter.harper@raspberrypi.com>

Author: peterharperuk

ports/rp2/pendsv.c

@@ -180,5 +180,7 @@ void PendSV_Handler(void) {
 
     #if MICROPY_PY_THREAD
     mp_thread_recursive_mutex_unlock(&pendsv_mutex);
+    // Check if a dispatch occurred while the interrupt was being serviced
+    pendsv_resume_run_dispatch();
     #endif
 }