Relaxed the memory ordering on the implementation of UnsafeArc
auto merge of #12962 : gereeter/rust/relaxed-arc, r=alexcrichton

The implementation of `UnsafeArc` was unnecessarily using stricter memory ordering than it needed to, producing worse performance in high contention.

I'm not exactly sure how to test or benchmark this change, as it should only show up rarely on a weakly ordered CPU while in contention.