When Mariano Rivera arrived at Mario’s on Arthur Avenue in the “Little Italy of The Bronx” for a fund-raiser for underprivileged kids whose fathers are in jail, he took one look at the banquet menu and quietly signaled owner Joe Migliucci that he’d prefer the eatery’s famed clams casino “for a starter, and some veal scaloppine, real thin, if you have it.” After his meal, the Yankee reliever went in the kitchen to thank the crew and then “signed everything in sight” for regular customers. “This guy is a star on and off the field,” Migliucci said.