Fix byte compilation of (eq foo 'default)

Do not use the symbol ‘default’ as a special marker.
Instead, use a value that cannot appear in the program,
improving on a patch proposed by Robert Cochran (Bug#31718#14).
* lisp/emacs-lisp/bytecomp.el (byte-compile--default-val):
New constant.
(byte-compile-cond-jump-table-info)
(byte-compile-cond-jump-table): Use it instead of 'default.
* test/lisp/emacs-lisp/bytecomp-tests.el:
(byte-opt-testsuite-arith-data): Add a test for the bug.

test/lisp/emacs-lisp/bytecomp-tests.el

@@ -289,7 +289,14 @@
             (t)))
     (let ((a))
       (cond ((eq a 'foo) 'incorrect)
-            ('correct))))
+            ('correct)))
+    ;; Bug#31734
+    (let ((variable 0))
+      (cond
+       ((eq variable 'default)
+	(message "equal"))
+       (t
+	(message "not equal")))))
   "List of expression for test.
 Each element will be executed by interpreter and with
 bytecompiled code, and their results compared.")