Supporting additional target for QNX or clarify in documentation

Following this tutorial:
https://doc.qt.io/qt-6/building-qt-for-qnx.html#creating-a-toolchain-file-for-qnx
the customer tries to build for "gcc_ntoaarch64le_gpp" and that fails. Obviously, we are not supporting that target well. So maybe we need to either add more target support in the future, or make it clear in documentation, e.g. "the only supported targets are..."

Fact: gpp is not a supported feature of Qt. So I'd close it for now until there is a really good reason to look into gpp.