i386: Mark NUMA support experimental

I did some testing and found quite a lot of problems (doesn't
boot at all on non NUMA and misassigns cores on Opteron systems).

Mark it as experimental and warn against its use for now.

It's still default y for SUMMIT/NUMAQ because it'll presumably
work on these systems.

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>