support for armv6

[davidhewitt]

Following a request at pydantic/pydantic-core#1721

... and a follow-up at pydantic/pydantic-core#1725

It looks like we don't yet support building for armv6 targets, I guess they would be

arm-unknown-linux-gnueabi
arm-unknown-linux-gnueabihf
arm-unknown-linux-musleabi
arm-unknown-linux-musleabihf

... I'm not sure whether the hf (hard float) targets are generally preferred, at least for the downstream pydantic-core request those seem to be the ones in use on raspberry pi. We can probably match whatever we do for armv7.