mt: Fix missing call to mtarch_pstop()
mt: Fix missing call to mtarch_pstop()

If mt_yield() needs it, then mt_exit() too. A thread could indeed be
preempted while calling mt_exit().

Signed-off-by: Benoît Thébaudeau <b3efeb72fa9d3102885911404c5fbece843f6c8f@gmail.com>

arurke/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki