parisc: Export __cmpxchg_u64 unconditionally

__cmpxchg_u64 is built and used outside CONFIG_64BIT and thus needs to
be exported. This fixes the following build error seen when building
parisc:allmodconfig.

ERROR: "__cmpxchg_u64" [drivers/net/ethernet/intel/i40e/i40e.ko] undefined!

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Helge Deller <deller@gmx.de>