i386: Rename ELF_MACHINE to be x86 specific

i386: Rename ELF_MACHINE to be x86 specific

Rename ELF_MACHINE to be I386 specific. This is used as-is by the
multiboot loader.

Linux-user previously used this definition but will not anymore,
falling back to the default bahaviour of using ELF_ARCH as ELF_MACHINE.

This removes another architecture specific definition from the global
namespace.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Acked-by: Eduardo Habkost <ehabkost@redhat.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>