standardize targets

Go doesn't have a "target" per se, so we do it manually on our side. In some places, it's all parts joined together, and sometimes we have an underline between each part.

Other builders use - to do it.

This might be a breaking change, but I think we should probably change our internal targets for Go to also split by -.

This would allow us to do things link strings.Split(target, "-") and then check if any of its parts is linux, for example.

The breaking changes this would introduce:

• manual targets in builds.targets (currently split by _, could make both work for the time being)
• what else?