sparc32: Add cmpxchg64().

commit 23198ddffb6cddb5d5824230af4dd4b46e4046a4 upstream.

This fixes the build with i40e driver enabled.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>