Possible/likely data race in QObjectData bit-field access

As discovered in https://codereview.qt-project.org/c/qt/qtbase/+/524176, QObjectData manages some of its data as a bit-field, and some of those bits (at least deleteLaterCalled) may be accessed from a different thread. Due to the nature of bit-fields, this may lead to data races, as we're not guarding every access to the bit-field.