percpu, x86: Avoid warnings of unused variables in per cpu

Avoid hundreds of warnings with a gcc 4.6 -Wall build.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>