elf-em.h: move EM_MICROBLAZE to the common header

The linux/audit.h header uses EM_MICROBLAZE in order to define
AUDIT_ARCH_MICROBLAZE, but it's only available in the microblaze
asm headers.  Move it to the common elf-em.h header so that the
define can be used on non-microblaze systems.  Otherwise we get
build errors that EM_MICROBLAZE isn't defined when we try to use
the AUDIT_ARCH_MICROBLAZE symbol.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>