qt4: fix 'arch' string due to microarchitecture change (#12957)

qt4 fails to build because it's expecting something like 'x86_64' rather 
than 'broadwell'.

var/spack/repos/builtin/packages/qt/package.py

@@ -427,7 +427,7 @@ def configure(self, spec, prefix):
             '-{0}gtkstyle'.format('' if '+gtk' in spec else 'no-'),
             '-{0}webkit'.format('' if '+webkit' in spec else 'no-'),
             '-{0}phonon'.format('' if '+phonon' in spec else 'no-'),
-            '-arch', str(spec.architecture.target),
+            '-arch', str(spec.target.family),
         ])
 
         # Disable phonon backend until gstreamer is setup as dependency