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/common/posix/posix_thread.c

@@ -262,6 +262,17 @@ os_cond_signal(korp_cond *cond)
     return BHT_OK;
 }
 
+int
+os_cond_broadcast(korp_cond *cond)
+{
+    assert(cond);
+
+    if (pthread_cond_broadcast(cond) != BHT_OK)
+        return BHT_ERROR;
+
+    return BHT_OK;
+}
+
 int
 os_thread_join(korp_tid thread, void **value_ptr)
 {