Need two backslashes in regex string.

The `auto-mode-alist` regexes in emacs-mode.el all end with "\'" (one backslash + single quote) but they need to end with "\\\'" (two backslashes + single quote).

The regexes are trying to match the empty string at the end of a regex (i.e., make sure that the file extension is at the end of the file name). Such a match is represented by either "$" or "\'". However, inside an elisp string the backslash has to be escaped/doubled.