a.out: restore CMAGIC

Part of UAPI and the on-disk format:
this means that it's not a magic number per magic-number.rst,
and it's best to leave it untouched to avoid breaking userspace
and suffer the same fate as a.out in general

Fixes: 53c2bd6790 ("a.out: remove define-only CMAGIC, previously magic number")
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Link: https://lore.kernel.org/r/20220926151554.7gxd6unp5727vw3c@tarta.nabijaczleweli.xyz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/uapi/linux/a.out.h

@@ -70,6 +70,9 @@ enum machine_type {
    The first page is unmapped to help trap NULL pointer references */
 #define QMAGIC 0314
 
+/* Code indicating core file.  */
+#define CMAGIC 0421
+
 #if !defined (N_BADMAG)
 #define N_BADMAG(x)	  (N_MAGIC(x) != OMAGIC		\
 			&& N_MAGIC(x) != NMAGIC		\