x86: Deprecate a.out support

Linux supports ELF binaries for ~25 years now.  a.out coredumping has
bitrotten quite significantly and would need some fixing to get it into
shape again but considering how even the toolchains cannot create a.out
executables in its default configuration, let's deprecate a.out support
and remove it a couple of releases later, instead.

Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Richard Weinberger <richard@nod.at>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: Jann Horn <jannh@google.com>
Cc: <linux-api@vger.kernel.org>
Cc: <linux-fsdevel@vger.kernel.org>
Cc: lkml <linux-kernel@vger.kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: <x86@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>