QEventLoop: fix potential UB in QBasicAtomic initialization

Until C++17 (inclusive), a default-constructed std::atomic object can,
officially, only be initialized with a call to std::atomic_init, for
which QBasicAtomic doesn't have API. It is even unclear whether
zero-initialization of static and thread-local objects will cause the
object to be initialized.

To fix, port from QBasicAtomic to QAtomic (which initializes to zero).

Task-number: QTBUG-137465
Pick-to: 6.10 6.9 6.8 6.5
Change-Id: I31a95b36506c376ef0817ef3d61bd8ca02371585
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qeventloop_p.h

@@ -34,8 +34,8 @@ public:
 
     QAtomicInt quitLockRef;
 
-    QBasicAtomicInt exit; // bool
+    QAtomicInt exit; // bool
-    QBasicAtomicInt returnCode;
+    QAtomicInt returnCode;
     bool inExec;
 
     void ref()