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 https://github.com/rust-lang/rust/pull/121841, another portion of https://github.com/rust-lang/rust/pull/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: https://github.com/rust-lang/rust/issues/116909

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

src/doc/unstable-book/src/language-features/f128.md

@@ -0,0 +1,9 @@
+# `f128`
+
+The tracking issue for this feature is: [#116909]
+
+[#116909]: https://github.com/rust-lang/rust/issues/116909
+
+---
+
+Enable the `f128` type for  IEEE 128-bit floating numbers (quad precision).

src/doc/unstable-book/src/language-features/f16.md

@@ -0,0 +1,9 @@
+# `f16`
+
+The tracking issue for this feature is: [#116909]
+
+[#116909]: https://github.com/rust-lang/rust/issues/116909
+
+---
+
+Enable the `f16` type for  IEEE 16-bit floating numbers (half precision).