added support for pushing 20-bit to stack for interrupts
added support for pushing 20-bit to stack for interrupts

contiki-os/mspsim,nfi/mspsim,joakimeriksson/mspsim,cmorty/mspsim,mspsim/mspsim,fvdnabee/mspsim,cmorty/mspsim,contiki-os/mspsim,ransford/mspsim,nfi/mspsim,mspsim/mspsim,fvdnabee/mspsim,ransford/mspsim,joakimeriksson/mspsim