Arch support is broken

Although the basic infrastructure for different Arch support is there, it totally doesn't work right now. Things to fix include:
- make should_build depend on Arch
- split the Python installation by Arch (or only allow one Arch per dist/apk?)
- fix all existing hardcoded Archs to be properly general
- somehow load only the right .so files for the Arch (needs extra code to handle libpymodules.so?)
