Go-mode in Emacs 23 does not recognize a backslash followed
by a backquote as end of raw string literal, as it does not
support syntax-propertize-function which Go-mode uses to
remove special meaning from backslashes in ``.

This patch provides a fallback mechanism to do the same thing
using font-lock-syntactic-keywords, which is supported by
Emacs 23.

LGTM=dominik.honnef
R=golang-codereviews, dominik.honnef
CC=adonovan, golang-codereviews
https://golang.org/cl/78730048