py/scheduler: De-inline and fix race with pending exception / scheduler.

The optimisation that allows a single check in the VM for either a pending
exception or non-empty scheduler queue doesn't work when threading is
enabled, as one thread can clear the sched_state if it has no pending
exception, meaning the thread with the pending exception will never see it.

This removes that optimisation for threaded builds.

Also fixes a race in non-scheduler builds where get-and-clear of the
pending exception is not protected by the atomic section.

Also removes the bulk of the inlining of pending exceptions and scheduler
handling from the VM. This just costs code size and complexity at no
performance benefit.

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

py/runtime.h

@@ -76,7 +76,6 @@ void mp_deinit(void);
 void mp_sched_exception(mp_obj_t exc);
 void mp_sched_keyboard_interrupt(void);
 void mp_handle_pending(bool raise_exc);
-void mp_handle_pending_tail(mp_uint_t atomic_state);
 
 #if MICROPY_ENABLE_SCHEDULER
 void mp_sched_lock(void);