Fix QBasicAtomicPointer::{load,store} to actually be relaxed

We were using direct loading and operator=, which for everything except
std::atomic was very relaxed. But std::atomic<T *> defines the direct
access to actually be the least relaxed possible, under the idea that if
you didn't know any better to use a member function, you probably need
the most protection.

So use Ops::load and Ops::store.

Change-Id: Id5480807d25e49e78b79ffff144a06a2e6398576
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>