Improve Edebug error for attempting to instrument built-in functions.

* lisp/emacs-lisp/edebug.el (edebug-instrument-function): Use it to
signal an error for built-in functions.

* lisp/emacs-lisp/find-func.el (find-function-noselect): New arg
lisp-only.

Fixes: debbugs:6664

lisp/emacs-lisp/edebug.el

@@ -3408,7 +3408,7 @@ go to the end of the last sexp, or if that is the same point, then step."
       (message "%s is already instrumented." func)
       func)
      (t
-      (let ((loc (find-function-noselect func)))
+      (let ((loc (find-function-noselect func t)))
 	(unless (cdr loc)
 	  (error "Could not find the definition in its file"))
 	(with-current-buffer (car loc)