Run pthread_atfork only once

Close #7704

native/src/core/thread.cpp

@@ -42,8 +42,6 @@ static void reset_pool() {
 }
 
 static void *thread_pool_loop(void * const is_core_pool) {
-    pthread_atfork(nullptr, nullptr, &reset_pool);
-
     // Block all signals
     sigset_t mask;
     sigfillset(&mask);
@@ -83,6 +81,10 @@ static void *thread_pool_loop(void * const is_core_pool) {
     }
 }
 
+void init_thread_pool() {
+    pthread_atfork(nullptr, nullptr, &reset_pool);
+}
+
 void exec_task(function<void()> &&task) {
     mutex_guard g(lock);
     pending_task.swap(task);