Free the QFreeList object allocated memory on exit

This memory allocation was introduced in
314c83c0c2. With a compiler without thread
safe statics support mutex.cpp use a function named freelist() to create
the global QFreeList object. it will be created when the first time it was
accessed, but will never be released. This patch use Q_DESTRUCTOR_FUNCTION
to delete this object.

Task-number: QTBUG-48359
Change-Id: I4e4716930930aa98630101a1f96de6a7672af9cb
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qmutex.cpp

@@ -571,19 +571,26 @@ FreeList *freelist()
     return &list;
 }
 #else
+static QBasicAtomicPointer<FreeList> freeListPtr;
+
 FreeList *freelist()
 {
-    static QAtomicPointer<FreeList> list;
-    FreeList *local = list.loadAcquire();
+    FreeList *local = freeListPtr.loadAcquire();
     if (!local) {
         local = new FreeList;
-        if (!list.testAndSetRelease(0, local)) {
+        if (!freeListPtr.testAndSetRelease(0, local)) {
             delete local;
-            local = list.loadAcquire();
+            local = freeListPtr.loadAcquire();
         }
     }
     return local;
 }
+
+static void qFreeListDeleter()
+{
+    delete freeListPtr.load();
+}
+Q_DESTRUCTOR_FUNCTION(qFreeListDeleter)
 #endif
 }