fix fallback architecture for MSVC

Use x86 for a 32 bit build of qmake and x86_64 for 64 bit.
This is needed for shells that do not set VCINSTALLDIR.

Change-Id: I0843c1a590161669530b99f45ab59d523e6596c3
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

qmake/library/qmakeevaluator.cpp

@@ -962,7 +962,11 @@ static ProString msvcBinDirToQMakeArch(QString subdir)
 
 static ProString defaultMsvcArchitecture()
 {
+#if defined(Q_OS_WIN64)
+    return ProString("x86_64");
+#else
     return ProString("x86");
+#endif
 }
 
 static ProString msvcArchitecture(const QString &vcInstallDir, const QString &pathVar)