Auto merge of #121926 - tgross35:f16-f128-step3-feature-gate, r=compiler-errors,petrochenkov

`f16` and `f128` step 3: compiler support & feature gate

Continuation of rust-lang/rust#121841, another portion of rust-lang/rust#114607

This PR exposes the new types to the world and adds a feature gate. Marking this as a draft because I need some feedback on where I did the feature gate check. It also does not yet catch type via suffixed literals (so the feature gate test will fail, probably some others too because I haven't belssed).

If there is a better place to check all types after resolution, I can do that. If not, I figure maybe I can add a second gate location in AST when it checks numeric suffixes.

Unfortunately I still don't think there is much testing to be done for correctness (codegen tests or parsed value checks) until we have basic library support. I think that will be the next step.

Tracking issue: rust-lang/rust#116909

r? `@compiler-errors`
cc `@Nilstrieb`
`@rustbot` label +F-f16_and_f128

Author: bors