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: 53c2bd67 ("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>