s390: Remove ELF_MACHINE from cpu.h

s390: Remove ELF_MACHINE from cpu.h

The bootloader can just pass EM_S390 directly, as that
is architecture specific code.

This removes another architecture specific definition from the global
namespace.

Cc: Richard Henderson <rth@twiddle.net>
Cc: Alexander Graf <agraf@suse.de>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Acked-By: Riku Voipio <riku.voipio@linaro.org>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>