[Kernel] Retain handle in ExCreateThread. Game closing a created thread's handle shouldn't result in its handle being freed.

src/xenia/kernel/xboxkrnl/xboxkrnl_threading.cc

@@ -136,6 +136,7 @@ dword_result_t ExCreateThread(lpdword_t handle_ptr, dword_t stack_size,
       if (creation_flags & 0x80) {
         *handle_ptr = thread->guest_object();
       } else {
+        thread->RetainHandle();
         *handle_ptr = thread->handle();
       }
     }