x86: Generate cmpxchg build failures

Rework the x86 cmpxchg() implementation to generate build failures
when used on improper types.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
LKML-Reference: <1254771187.21044.22.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>