Makefile: Create dynamic build targets

[gogolok]

Summary

Use a Makefile function to create dynamic build targets so it's simple to add new GOOS/GOARCH combinations if required. The duplication of Makefile code is reduced.

Related

#1171

Context

My 'overall goal' is still to get some FreeBSD support as attempted in #1271 and #1087 .

I thought it might make sense to help out with #1171 and come up with a simpler Makefile so that other GOOS/GOARCH can be added more easily. I'm not a Makefile expert.

I've left windows out since it might get removed soon.
I've left out the darwin build targets for the time being.

Other Makefile targets like the package targets can be implemented in a similar way once this attempt is accepted of being worth it.

I wonder (and this contradicts this attempt in this PR) whether the Makefile should just work with GOOS and GOARCH set outside the Makefile and the .github workflows to create releases should actually have several make build (or make package or ...) runs to create individual binaries/packages/whatsoever.

For this PR and the attempt at making the Makefile more dynamic I see the following advantages and disadvantages:

Advantages

• Code duplication is reduced.
• No separate *.mk files.

Disadvantages

• It might be harder to understand the new Makefile logic.
• It seems like there is no 'Makefile feature' to review the 'first pass' of the Makefile. It would be nice to see the new targets the new function actually generates. At least I'm not aware of such a Makefile feature.
• If you want to build binaries for currently unsupported GOOS/GOARCH, you would need to patch the Makefile (FreeBSD case for example at the moment).

[jabrown85]

Nice - I'm not sure which approach I prefer either. I think I'm fine with either approach having only seen the diff for the dynamic one here.

This PR will have to wait until we've release a last windows lifecycle release in the next week or two.