Add target armv6-unknown-linux-*eabihf

This would add full support for the early Raspberry Pi models: Raspberry Pi 1 and Zero, which are still widely in use. `uv` itself does have the `arm-unknown-linux-musleabihf` builds for this architecture already, but Python cannot be installed. Those slow RPi models would benefit particularly from pre-compiled Python builds.

Not sure about `gnueabihf` vs `musleabihf`, I guess best is to keep it in sync with `uv` and use `musleabihf`. IIRC `uv` is first looking for a Python build that matches its own triple.

However, the big problem is probably that there are no "official" maintained Raspbian base Docker images. There are some very old ones, though: https://hub.docker.com/u/raspbian
The best main ones are from tianon: https://hub.docker.com/r/tianon/raspbian
But all no verified publishers.
Balena stopped maintaining theirs: https://hub.docker.com/r/balenalib/rpi-raspbian