x86: make UV support configurable

Make X86 SGI Ultraviolet support configurable. Saves about 13K of text size
on my modest config.

   text    data     bss     dec     hex filename
6770537 1158680  694356 8623573  8395d5 vmlinux
6757492 1157664  694228 8609384  835e68 vmlinux.nouv

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>