Implement pthread_cond_broadcast wrapper for lib-pthread (#982)

Implement pthread_cond_broadcast wrapper for lib-pthread
- support pthread_cond_broadcast wrapper for posix/linux-sgx/windows
- update document for building multi-thread wasm app with emcc

core/shared/platform/include/platform_api_extension.h

@@ -179,6 +179,16 @@ os_cond_reltimedwait(korp_cond *cond, korp_mutex *mutex, uint64 useconds);
 int
 os_cond_signal(korp_cond *cond);
 
+/**
+ * Broadcast the condition variable
+ *
+ * @param cond condition variable
+ *
+ * @return 0 if success
+ */
+int
+os_cond_broadcast(korp_cond *cond);
+
 /****************************************************
  *                     Section 2                    *
  *                   Socket support                 *