You're doing the wrong thing. DNF has two properties you can use:
"basearch" and "arch". If you set the "arch" correctly, it will
resolve the "basearch" automatically. The reason it works with x86
arches is because the base arch is part of the arch name, which
appears to validate the incorrect assumption that base arches are
real. They are not. What you need to do is set the "arch" option to
the real architecture (i.e. the architecture rpm builds for) and let
it figure out base arch for you.