It has been a pleasure to help Mark Williams of Samsung to elaborate and test the the official guide that describes how to cross-compile Qt5 (using version 5.9.2) for ARTIK 710 using Ubuntu 16.04 on your development PC.
You have to prepare an ARTIK 710 sysroot to make sure that the compiler can find aarch64 libraries and header files.
The complete documentation link is here:
https://developer.artik.io/documentation/developer-guide/multimedia/qt5.html