Add support for IBM CP 850 to enable decoding of DOS executable outputs on Windows (QProcess)

IBM 850 unicode support is important to be able to print correctly QProcess outputs on Windows.
Qt5 had the support but Qt6 miss this support.

Maybe there is an undocumented way to have cross plartform printing of QProcess result in a correct way?

I discovered this bug when I wanted to print the result of CMake call from QProcess:

QProcess compileProcess;
compileProcess.start("cmake", {"--build", ".", "--config", "Release"});
compileProcess.waitForFinished();
QByteArray allStandardOutput = compileProcess.readAllStandardOutput();
// Convert allStandardOutput to QString using the IBM 850 or the good unicode encoding