Use the python:any syntax to pull build version of
python when cross-building qemu.

Enable building for arm64; the arm64 target is not yet merged, but the package doesn't need arm64 target support to build for an arm64 host.

Enable building for ppc64el (in both Debian and Ubuntu): Debian does not have a ppc64el port yet, but qemu builds out of the box there so it's safe/appropriate to enable.

add rule to make d/control only if d/control-in is present, and depend on d/rules (defining ${VENDOR}) too