QSemaphore: add minor optimization for 64-bit Linux systems

Since we won't use the high bit of the low 32-bit word at all, we don't
need the AND with 0x7fffffff either. Just cast.

Change-Id: I72f5230ad59948f784eafffd151aa5a7dee995db
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/thread/qsemaphore.cpp

@@ -140,6 +140,13 @@ static const quintptr futexNeedsWakeAllBit =
 static int futexAvailCounter(quintptr v)
 {
     // the low 31 bits
+    if (futexHasWaiterCount) {
+        // the high bit of the low word isn't used
+        Q_ASSERT((v & 0x80000000U) == 0);
+
+        // so we can be a little faster
+        return int(unsigned(v));
+    }
     return int(v & 0x7fffffffU);
 }