Auto merge of #121728 - tgross35:f16-f128-step1-ty-updates, r=compiler-errors

Add stubs in IR and ABI for `f16` and `f128`

This is the very first step toward the changes in https://github.com/rust-lang/rust/pull/114607 and the [`f16` and `f128` RFC](https://rust-lang.github.io/rfcs/3453-f16-and-f128.html). It adds the types to `rustc_type_ir::FloatTy` and `rustc_abi::Primitive`, and just propagates those out as `unimplemented!` stubs where necessary.

These types do not parse yet so there is no feature gate, and it should be okay to use `unimplemented!`.

The next steps will probably be AST support with parsing and the feature gate.

r? `@compiler-errors`
cc `@Nilstrieb` suggested breaking the PR up in https://github.com/rust-lang/rust/pull/120645#issuecomment-1925900572

compiler/rustc_span/src/symbol.rs

@@ -758,6 +758,8 @@ symbols! {
         external,
         external_doc,
         f,
+        f128,
+        f16,
         f16c_target_feature,
         f32,
         f32_legacy_const_digits,