AVR would be nice. I’m fond of those weird little guys, they can take a beating. But mostly, yes.
But let’s also note that @uint (welcome to the forum!) refers to “chip vendor adaptations”, and that’s rather a different thing. There’s a long tail there, and yeah, probably it’s LLVM or nothing for a lot of that stuff.
This can be achieved in one way only: have several compiler backends to test output against. Otherwise Hyrum’s Law kicks in, and the code will be dependent on e.g. LLVM quirks, no matter how badly we might want it not to be.
Look at everything which went into making it possible to compile the Linux kernel with clang, and not just gcc. Sure they’re both “C compilers” but, details matter.