Make _emscripten_run_callback_on_thread asynchronous

...fixing a regression introduced in c41f659
"Remove use of legacy proxy API from library_html5.js. NFC (emscripten-core#20370)".

Fixes: emscripten-core#22442

(cherry-picked from 17f1bb7 at
<emscripten-core#22443> "Make
_emscripten_run_callback_on_thread asynchronous")

system/lib/html5/callback.c

@@ -21,6 +21,7 @@ typedef struct callback_args_t {
 static void do_callback(void* arg) {
   callback_args_t* args = (callback_args_t*)arg;
   args->callback(args->event_type, args->event_data, args->user_data);
+  free(arg);
 }
 
 void _emscripten_run_callback_on_thread(pthread_t t,
@@ -29,14 +30,13 @@ void _emscripten_run_callback_on_thread(pthread_t t,
                                         void* event_data,
                                         void* user_data) {
   em_proxying_queue* q = emscripten_proxy_get_system_queue();
-  callback_args_t arg = {
-    .callback = f,
-    .event_type = event_type,
-    .event_data = event_data,
-    .user_data = user_data,
-  };
+  callback_args_t* arg = malloc(sizeof(callback_args_t));
+  arg->callback = f;
+  arg->event_type = event_type;
+  arg->event_data = event_data;
+  arg->user_data = user_data;
 
-  if (!emscripten_proxy_sync(q, t, do_callback, &arg)) {
-    assert(false && "emscripten_proxy_sync failed");
+  if (!emscripten_proxy_async(q, t, do_callback, arg)) {
+    assert(false && "emscripten_proxy_async failed");
   }
 }