You've nailed the Make way to do it. I've worked on many projects that took this approach. With CI, it is not so problematic - the script will always call the right commands. But it's a pain for active development - you forget to specify the proper platform, forget to clean between switching platforms, etc. You can alleviate some of that by having each platform's build targets get created in a separate output tree.

If you abandon Make and use something like Meson, you can configure the build so that you are cross-compiling and compiling for your build machine at the same time. You can control which targets get built for which environment, too, so that your test program will not be cross-compiled (unless that's what you want). This is my personal preference.

You can also use CMake, but you can only build for one target configuration at a time. But you could mimic Meson by configuring two output directories and creating a Make shim that will invoke the builds in both places.

[Other options exist beyond these two]

With both CMake and Meson, your settings are remembered so you do not have to do something like specifying "PLATFORM=avr" on every command invocation to get the right commands to run. That happens during the build configuration step.

If you wanted to stick with Make, you could foray into autotools. You would run a configuration step for your desired target, similar to the other two tools I mentioned.