thread: SDL_CreateThread() shouldn't return before the new thread is set up.

Fixes #15290.

(cherry picked from commit 0756603e6d)

src/thread/SDL_thread_c.h

@@ -54,6 +54,7 @@ struct SDL_Thread
     SDL_error errbuf;
     char *name;
     size_t stacksize; // 0 for default, >0 for user-specified stack size.
+    SDL_Semaphore *ready_sem;  // signals when the thread is set up and about to start running.
     int(SDLCALL *userfunc)(void *);
     void *userdata;
     void *data;