qmake WinRT: Default to x86 for lib path

If VCPROJ_ARCH is not recognized or unset, make "arch" default to x86,
or link won't find the libs.

Change-Id: If2cbda37a80c0fa43e1464775c036cebf10f931a
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

qmake/generators/win32/msvc_nmake.cpp

@@ -123,6 +123,8 @@ NmakeMakefileGenerator::writeMakefile(QTextStream &t)
                 } else if (arch == QStringLiteral("x64")) {
                     compiler = QStringLiteral("x86_amd64");
                     compilerArch = QStringLiteral("amd64");
+                } else {
+                    arch = QStringLiteral("x86");
                 }
 
                 const QString msvcVer = project->first("MSVC_VER").toQString();