[fuzzer][wasm] Use externref in structs, locals and signatures

Change-Id: I57c805b899a25e58035f2ed9accd10e43114fd80
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3802689Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Auto-Submit: Matthias Liedtke <mliedtke@chromium.org>
Commit-Queue: Matthias Liedtke <mliedtke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82112}

test/fuzzer/wasm-compile.cc

@@ -126,7 +126,7 @@ ValueType GetValueTypeHelper(DataRange* data, bool liftoff_as_reference,
   const bool nullable =
       (allow_non_nullable == kAllowNonNullables) ? data->get<bool>() : true;
   if (nullable) {
-    types.insert(types.end(), {kWasmI31Ref, kWasmFuncRef});
+    types.insert(types.end(), {kWasmI31Ref, kWasmFuncRef, kWasmExternRef});
   }
   if (include_generics == kIncludeGenerics) {
     types.insert(types.end(), {kWasmDataRef, kWasmAnyRef, kWasmEqRef});