When cross-compiling a plugin to Darwin you must set the environment variable CC to a Darwin cross-compiler. Building a plugin requires invoking the system linker.

OK... Where can I get information about this? https://github.com/golang/go/wiki/WindowsCrossCompiling says that I just set GOOS and GOARCH and everything will work.

What do I put in CC? What counts as a Darwin cross-compiler? Why does invoking the system linker change things? Why does it spit out an error that makes it look like go itself has a bug rather than an informative message?

That page needs an overhaul. I added a few words.

Cross compiling a pure Go program works as described on that page. But cross-compiling a program that uses cgo, or that uses the build modes c-shared, c-archive, shared, or plugin, requires a C cross-compiler. A cross-compiler is a compiler that runs on your host system (in your case, GNU/Linux) and can compile a C program into a compiler that runs on your target system (in your case, macOS).

Invoking the system linker changes things because you need to use a linker that can produce an executable that runs on the target system.

I agree that the error message could likely be improved. That is #16395.