EM_MOXIE now has a proper assigned elf code. Use it. Register the old
interim value as EM_MOXIE_OLD and accept either in elf loading.

Cc: Anthony Green <green@moxielogic.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>