or32: Remove ELF_MACHINE from cpu.h

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

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

This removes another architecture specific definition from the global
namespace.

Cc: Jia Liu <proljc@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>