WASHINGTON — Hillary Rodham Clinton’s desperate attempt to soften her image is taking her back to the stage of “Saturday Night Live,” where she hopes to reverse her falling poll numbers with self-deprecating jokes about pantsuits. Her sketch this week is another in a string of recent lighthearted appearances on such programs as “The Tonight Show” and “The Ellen DeGeneres Show” that Clinton has used to try to counter the endless drumbeat of negative stories about her private email server and polls questioning her trustworthiness.