thread: Remove semaphore in thread creation.

This was added so that the new thread definitely has its threadid set, via
SDL_GetCurrentThreadID(), before SDL_CreatThread returns, but this broke
Emscripten, which can't wait on a newly-created thread, since the thread won't
start until a later mainloop iteration.

Now we have the creating thread set this id in SDL_SYS_CreateThread, where
platform-specific logic can figure out how to calculate the new thread's ID
from the parent thread, without using SDL_GetCurrentThreadID().

Fixes #15509.

(cherry picked from commit 922b872b4f)

src/thread/pthread/SDL_systhread.c

@@ -116,6 +116,8 @@ bool SDL_SYS_CreateThread(SDL_Thread *thread,
         return SDL_SetError("Not enough resources to create thread");
     }
 
+    thread->threadid = (SDL_ThreadID) thread->handle;  // the SDL thread ID is just the pthread_t.
+
     return true;
 }