I just was lazy and did a 'apt list *libqt5* to find all the qt5 related libraries which were deployed by Ubuntu.
We are running the apps from the terminal and ensure that the LD_LIBRARY_PATH is targetting the Qt 5.15 and the ArcGIS Runtime for Qt directory.
export LD_LIBRARY_PATH=~/Qt/5.15.2/gcc_64/lib:~/arcgis/runtime_sdk/qt100.11/sdk/linux/x64/lib
We attached the ldd -r output and attached the gdb 'info sharedlibrary' output. The gdb shows, that at least the correct Qt and runtime libraries are loaded during execution. But when we set a breakpoint at main using 'break main', the breakpoint is not reached. As far as we can judge it, the libruntimecore.so try to load some library or is missing something on this OS causing the SIGILL.
For the simple Qt Quick App without using ArcGIS Runtime we had to install the necessary OpenGL dev libraries, too.
sudo apt-get install libxcb-randr0-dev libxcb-xtest0-dev libxcb-xinerama0-dev libxcb-shape0-dev libxcb-xkb-dev
Is it possible to get some debug symbols for the libruntimecore.so?