no-thread: add QThread::isMainThread() definition

In this configuration there is one thread (as far as Qt is
concerned), and that is the main thread.

Pick-to: 6.8
Change-Id: I6817961a0f95f2e8b1bb3ea120154d954f1eaf0f
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Even Oscar Andersen <even.oscar.andersen@qt.io>
(cherry picked from commit 59039f42d8)
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/thread/qthread.cpp

@@ -1010,6 +1010,11 @@ QThread *QThread::currentThread()
     return QThreadData::current()->thread.loadAcquire();
 }
 
+bool QThread::isMainThread() noexcept
+{
+    return true;
+}
+
 bool QThread::isCurrentThread() const noexcept
 {
     return true;