Implement a minimal QTimezonePrivate supporting Qt::LocalTime as system zone

Prompted by a comment from lpotter on QTBUG-95314: it should be possible to implement a time-zone backend that consults the same system APIs that QDateTime currently uses for its conversions between local time and UTC. This implements only one time-zone, the system time-zone, and can probably only offer its abbreviation, no full name.
It could then be used, in conjunction with the existing UTC-offset backend, as fall-back backend on systems that lack a reliable source of timezone information.

It may be possible to do this in a way that moves the relevant QDteTime code to this new backend, so that QDateTime can access it privately, even when it's not exposed as the implementation of the system time-zone, so as to unify local-time and zone-time into a common code-path in assorted QDateTime methods.