x86_64: x86_64 - Use non locked version for local_cmpxchg()

local_cmpxchg() should not use any LOCK prefix.  This change probably
got lost in the move to cmpxchg.h.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Acked-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>