x86, percpu: Use ASM_NOP4 instead of hardcoding P6_NOP4

For use in assembly constants, use the ASM_NOP* defines.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/1303166160-10315-2-git-send-email-hpa@linux.intel.com