Fix a panic with Func::new and reference types (#2039)

Currently `Func::new` will panic if one of the arguments of the function
is a reference type and the `Store` doesn't have reference types
enabled. This happens because cranelift isn't configure to enable stack
maps but the register allocators expects them to exist when reference
types are seen.

The fix here is to always enable reference types in cranelift for our
trampoline generation and `Func::new`. This should hopefully ensure that
trampolines are generated correctly and they'll just not be able to get
hooked up to an `Instance` because validation will prevent reference
types from being used elsewhere.

tests/all/func.rs

@@ -507,3 +507,18 @@ fn pass_cross_store_arg() -> anyhow::Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn externref_signature_no_reference_types() -> anyhow::Result<()> {
+    let store = Store::default();
+    Func::wrap(&store, |_: Option<Func>| {});
+    Func::new(
+        &store,
+        FuncType::new(
+            Box::new([ValType::FuncRef, ValType::ExternRef]),
+            Box::new([ValType::FuncRef, ValType::ExternRef]),
+        ),
+        |_, _, _| Ok(()),
+    );
+    Ok(())
+}