test/ring: relax memory ordering for stress test

test/ring: relax memory ordering for stress test

wrk_cmd variable is used to signal the worker thread to start
or stop the stress test loop. Relaxed barriers are used
to achieve the same.

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Ola Liljedahl <ola.liljedahl@arm.com>
Reviewed-by: Feifei Wang <feifei.wang2@arm.com>