arm64: add tst_atomic_add_return

The patch fixes build of LTP, at least with gcc 5.3.
The implementation is taken from kernel v4.9.

Signed-off-by: Yury Norov <ynorov@caviumnetworks.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>
Acked-by: Jan Stancek <jstancek@redhat.com>
1 file changed