Donald Trump has won the backing of a legendary New York Republican — who also blasted his own state party’s failure to rally behind the candidacy of its “favorite son’’ presidential hopeful. “You have in Donald Trump a tremendous New York businessman with thousands of employees, a man who helped remake Manhattan for the better, and one who was always there with a contribution when we needed him to back us,†former state GOP Chairman Bill Powers told The Post. Powers said he feared Trump was already angry enough at the New York GOP that if he becomes president he’ll...