lm32: Remove ELF_MACHINE from cpu.h

lm32: Remove ELF_MACHINE from cpu.h

The bootloaders can just pass EM_LATTICEMICO32 directly, as that is
architecture specific code.

This removes another architecture specific definition from the global
namespace.

Cc: Michael Walle <michael@walle.cc>
Acked-By: Michael Walle <michael@walle.cc>
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>