I did start to try to take clang's TargetInfo code (https://github.com/llvm/llvm-project/blob/main/clang/lib/Bas...) and porting it over to TableGen, primarily so somebody could actually extract useful auto-generated documentation out of it, like "What are all the targets available?"

I actually do have working code for the triple-to-TargetInfo instantiation portion (which is fun because there's one or two cases that juuuust aren't quite like all of the others, and I'm not sure if that's a bad copy-paste job or actually intentional). But I never got around to working out how to actually integrate the actual bodies of TargetInfo implementations--which provide things like the properties of C/C++ fundamental types or default macros--into the TableGen easily, so that patch is still merely languishing somewhere on my computer.