SVC: Implemented GetThreadId.

For now threads are using their Handle value as their Id, it should not really cause any problems because Handle values are unique in Citra, but it should be changed. I left a ToDo there because this is not correct behavior as per hardware.

src/core/hle/kernel/thread.h

@@ -58,6 +58,9 @@ void Reschedule();
 /// Stops the current thread
 ResultCode StopThread(Handle thread, const char* reason);
 
+// Retrieves the thread id of the specified thread handle
+ResultCode GetThreadId(u32* thread_id, Handle handle);
+
 /// Resumes a thread from waiting by marking it as "ready"
 void ResumeThreadFromWait(Handle handle);