PYONGYANG, North Korea — North Koreans are not exactly used to seeing 300-pound Americans in tiny black underpants with “strong man” emblazoned across the rear. So they audibly gasped when Jon Andersen, the professional wrestler from San Francisco with muscles that look set to burst out of his skin, appeared on a floodlit stage in a Pyongyang stadium Saturday night, with music declaring, “He’s a macho man,” blaring from the sound system.