Rename ELF_MACHINE to be PPC specific. This is used as-is by the
various PPC bootloaders and is locally defined to ELF_MACHINE in linux
user in PPC specific ifdeffery.

This removes another architecture specific definition from the global
namespace (as desired by multi-arch).

Cc: Alexander Graf <agraf@suse.de>
Cc: qemu-ppc@nongnu.org
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>