percpu, x86: Add arch-specific this_cpu_cmpxchg_double() support

Support this_cpu_cmpxchg_double() using the cmpxchg16b and cmpxchg8b
instructions.

-tj: s/percpu_cmpxchg16b/percpu_cmpxchg16b_double/ for consistency and
     other cosmetic changes.

Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

arch/x86/lib/Makefile

@@ -42,4 +42,5 @@ else
         lib-y += memmove_64.o memset_64.o
         lib-y += copy_user_64.o rwlock_64.o copy_user_nocache_64.o
 	lib-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) += rwsem_64.o
+	lib-y += cmpxchg16b_emu.o
 endif