Support f16 and f128

[tgross35]

Thanks for filing a feature request! Please fill out the TODOs below.

Feature

Add support for 16-bit and 128-bit IEEE floating point numbers.

Benefit

This will allow using these types with rustc_codegen_clif, as described at rust-lang/rustc_codegen_cranelift#1461

Implementation

Unsure

Alternatives

Unsure, maybe it would be possible to stub out these types if adding them is difficult.