mb: Remove ELF_MACHINE from cpu.h

mb: Remove ELF_MACHINE from cpu.h

The only generic code relying on this is linux-user, but linux-users'
default behaviour or setting ELF_MACHINE to ELF_ARCH will handle this.

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

This removes another architecture specific definition from the global
namespace.

Cc: Edgar E. Iglesias <edgar.iglesias@gmail.com>
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>