[REGR:5->6] Performance regression in QLocale on macOS

On macOS with qt6 there is a performance regression in the method QLocale::toString(QDate).

macOS qt5: 0.000762 msecs per iteration
macOS qt6:  0.32 msecs per iteration

Test code is (see also attached project):

QLocale locale = QLocale::system();    
QDate curDate = QDate::currentDate();    
QBENCHMARK(locale.toString(curDate, "dd.MM.yyyy"));