SPRINGFIELD — Illinois’ top lawyer has jumped into a national spat over workers’ ability to band together against employers.Attorney General Lisa M. Madigan this week signed onto arguments at the U.S. Supreme Court that say prohibiting employees from jointly pursuing work-related claims in arbitration violates federal law.On Thursday, she joined attorneys general of 15 other states and the District of Columbia in a friend-of-the-court brief that says arbitration agreements forcing workers to waive the …