QThread: use load/store operations with explicit ordering

The implicit load is an acquire, which isn't necessary in any of these
cases, including the two liens changing the store: they were creating a
temporary QAtomicPointer, which introduced the loadAcquire().

I've left the qCDebug() lines alone.

Pick-to: 6.5
Change-Id: If45e068eaaf3cd4d2c81fffd1459a779b4eb4110
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 6806391)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>