cpu/native: improve `getpid()` stub message
cpu/native: improve `getpid()` stub message

jasonatran/RIOT,OlegHahm/RIOT,kYc0o/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,OlegHahm/RIOT,kaspar030/RIOT,ant9000/RIOT,kYc0o/RIOT,kYc0o/RIOT,kaspar030/RIOT,jasonatran/RIOT,miri64/RIOT,kaspar030/RIOT,miri64/RIOT,RIOT-OS/RIOT,miri64/RIOT,ant9000/RIOT,OlegHahm/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,miri64/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,ant9000/RIOT,kYc0o/RIOT,ant9000/RIOT,OlegHahm/RIOT,ant9000/RIOT,miri64/RIOT,RIOT-OS/RIOT,jasonatran/RIOT