type pun rb_f_notimplement

Other functions are already type-punned elsewhere.  rb_f_notimplement is
the only exceptional function that appear literally.  We have to take
care of it by hand.

vm_method.c

@@ -372,7 +372,7 @@ void
 rb_add_method_cfunc(VALUE klass, ID mid, VALUE (*func)(ANYARGS), int argc, rb_method_visibility_t visi)
 {
     if (argc < -2 || 15 < argc) rb_raise(rb_eArgError, "arity out of range: %d for -2..15", argc);
-    if (func != rb_f_notimplement) {
+    if (func != (VALUE(*)(ANYARGS))rb_f_notimplement) {
         rb_method_cfunc_t opt;
         opt.func = func;
         opt.argc = argc;
@@ -561,7 +561,7 @@ rb_method_definition_set(const rb_method_entry_t *me, rb_method_definition_t *de
             RB_OBJ_WRITE(me, &def->body.bmethod.defined_ractor, rb_ractor_self(GET_RACTOR()));
             return;
           case VM_METHOD_TYPE_NOTIMPLEMENTED:
-            setup_method_cfunc_struct(UNALIGNED_MEMBER_PTR(def, body.cfunc), rb_f_notimplement_internal, -1);
+            setup_method_cfunc_struct(UNALIGNED_MEMBER_PTR(def, body.cfunc), (VALUE(*)(ANYARGS))rb_f_notimplement_internal, -1);
             return;
           case VM_METHOD_TYPE_OPTIMIZED:
             def->body.optimized = *(rb_method_optimized_t *)opts;