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: NRichard Henderson <rth@twiddle.net>
Acked-By: NRiku Voipio <riku.voipio@linaro.org>
Signed-off-by: NPeter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>