Sanitise linux/audit.h for userspace consumption, split elf-em.h from elf.h

Don't include <linux/sched.h> outside __KERNEL__, and split the EM_xxx
definitions out of elf.h into elf-em.h so that audit.h can include just
that and not pollute the namespace any further than it needs to.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>