sparc: Remove ELF_MACHINE from cpu.h

sparc: Remove ELF_MACHINE from cpu.h

The bootloaders can just pass EM_SPARC or EM_SPARCV9 directly, as
they are architecture specific code (to one or the other).

This removes another architecture specific definition from the global
namespace.

Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
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>