It's been something like 3 years since I've worked in AOSP but IIRC mingw is for windows builds of adb. Someone else called out that AVR stuff is for ADK. The Android build tree is more than Android itself. It also builds includes a lot of the supporting tools and libraries for developers.

For avr-libc, probably this: http://tool.oschina.net/uploads/apidocs/android/guide/topics...

Unsure about mingw/go, but they are likely deps for some Android ecosystem project.

FWIW things may not actually be used anymore, doesn't mean the repo is ever taken down. Usually the manifests that sync these repo's simply "unhook" the repos when they're no longer in use. OP is excited that there's a bunch of links, but that's not the correct measure of "in use."