x86: voyager don't try to support uniprocessor builds

A while ago Randy Dunlap and Adrian Bunk suggested we simply prevent UP
voyager building.  I resisted this on the grounds that the nagging was the
only thing that was going to cause me to look at this.  However, now I
think we should probably take this course.

Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>