wireguard: Kconfig: select CRYPTO_CHACHA_S390

Select the new implementation of CHACHA20 for S390 when available.
It is faster than the generic software implementation, but also prevents
some linker errors in certain situations.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/linux-kernel/202207030630.6SZVkrWf-lkp@intel.com/


Signed-off-by: Vladis Dronov <vdronov@redhat.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>