Do not use Q_GLOBAL_STATIC in the implementation of QMutex

Since Q_GLOBAL_STATIC might use QMutex and cause a stack overflow.

Task-number: QTBUG-47554
Change-Id: I4853c5e9b9168d4a417200e2a45a1bf9cb103a30
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Pierre Rossi <pierre.rossi@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qmutex.cpp

@@ -563,7 +563,28 @@ const int FreeListConstants::Sizes[FreeListConstants::BlockCount] = {
 };
 
 typedef QFreeList<QMutexPrivate, FreeListConstants> FreeList;
-Q_GLOBAL_STATIC(FreeList, freelist);
+// We cannot use Q_GLOBAL_STATIC because it uses QMutex
+#if defined(Q_COMPILER_THREADSAFE_STATICS)
+FreeList *freelist()
+{
+    static FreeList list;
+    return &list;
+}
+#else
+FreeList *freelist()
+{
+    static QAtomicPointer<FreeList> list;
+    FreeList *local = list.loadAcquire();
+    if (!local) {
+        local = new FreeList;
+        if (!list.testAndSetRelease(0, local)) {
+            delete local;
+            local = list.loadAcquire();
+        }
+    }
+    return local;
+}
+#endif
 }
 
 QMutexPrivate *QMutexPrivate::allocate()