Add v0 symbol mangling for f16 and f128

As discussed at <#122106>, use the
crate encoding to represent new primitives.

compiler/rustc_symbol_mangling/src/v0.rs

@@ -319,11 +319,10 @@ impl<'tcx> Printer<'tcx> for SymbolMangler<'tcx> {
             ty::Uint(UintTy::U64) => "y",
             ty::Uint(UintTy::U128) => "o",
             ty::Uint(UintTy::Usize) => "j",
-            // FIXME(f16_f128): update these once `rustc-demangle` supports the new types
-            ty::Float(FloatTy::F16) => unimplemented!("f16_f128"),
+            ty::Float(FloatTy::F16) => "C3f16",
             ty::Float(FloatTy::F32) => "f",
             ty::Float(FloatTy::F64) => "d",
-            ty::Float(FloatTy::F128) => unimplemented!("f16_f128"),
+            ty::Float(FloatTy::F128) => "C4f128",
             ty::Never => "z",
 
             // Placeholders (should be demangled as `_`).

src/doc/rustc/src/symbol-mangling/v0.md

@@ -739,6 +739,8 @@ The type encodings based on the initial tag character are:
   * `z` — `!`
   * `p` — [placeholder] `_`
 
+Remaining primitives are encoded as a crate production, e.g. `C4f128`.
+
 * `A` — An [array][reference-array] `[T; N]`.
 
   > <span id="array-type">array-type</span> → `A` *[type]* *[const]*