ibus-libzhuyin cannot be cross built from source, because it
build-depends on a host architecture Python interpreter, which cannot be
installed. ibus-libzhuyin actually wants to run the interpreter during
build, so it likely needs a build architecture one. This can be achieved
by annotating the dependency with :native. Once doing so, ibus-libzhuyin
cross builds successfully. I'm attaching a patch for your convenience.

Helmut