A Norristown man has been ordered held over for trial on robbery and gun charges after a preliminary hearing on Friday.    Investigators say 20-year-old Mark Smith held up the 7-11 store on East Johnson Highway last month.   A store clerk testified at the hearing that it was Clark who put a gun to his head and forced him to open a cash register.   Clark allegedly got away with $100, a pack of cigarettes and an energy drink.   The night after the robbery, Smith and another man were wounded in a seperate shooting incident on Chestnut Street in Norristown.