OPTIM: ring: use relaxed stores to release the threads

OPTIM: ring: use relaxed stores to release the threads

We don't care in what order the threads are released, so we can write
their sent value using relaxed atomic stores. This brings a 3-5% perf
boost on ARM with 80 cores, reaching 7.25M/s, and doesn't change
anything on x86 since it keeps using strict ordering.