As of rust-lang/rust#121728, the backends now have stubs for f16 and f128: https://github.com/rust-lang/rust/blob/6cbf0926d54c80ea6d15df333be9281f65bbeb36/compiler/rustc_codegen_gcc/src/type_.rs#L123-L137.

It will be a while before these codepaths can even be exercised since the frontend isn't hooked up yet, (frontend is done now) but at some point this should be changed to either gracefully fail or do the right thing.