WIP: Makefiles: Add convenience clean targets

As per the suggestion offered in #20311, this patch adds a convenience <CMake target>/clean target to Makefiles to allow for per-CMake-target clean at the top level (without needing to dig into the CMake internal directory structure).

I would like to add some tests that verify the behavior, but as an amateur CMake developer I'm not sure how/where I would add them. Some guidance would be great!

Fixes: #20311
Topic-rename: makefile-clean-targets