There’s an old joke about two brothers lined up before a firing squad, about to be executed as prisoners of war. When the younger, more defiant one is offered his last cigarette, he spits in the commanding officer’s face, exclaiming: “I don’t need your lousy cigarette you stinkin’ fascist S.O.B.!”At which point his older, cautious brother, admonishes him with: “Julio! Don’t make trouble.”That about sums up the theme of Salvadoran-American playwright Exal Iraheta’s “Last Hermanos” (the Last Brothers), playing through June …