macOS: Pass multi arch flags when linking qmake

Amends e8946771862bcf3efc59b8edca0b9974afff8d1a.

We were building all the sources with multiple archs, but not when
linking the final executable.

Change-Id: I4f1110ff42bc5118b775761b6719f42b09622d40
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

configure

@@ -787,6 +787,7 @@ setBootstrapVariable()
             echo "EXTRA_CXXFLAGS += -MMD" >> "$mkfile"
             for arch in $QMAKE_APPLE_DEVICE_ARCHS; do
                 echo "EXTRA_CXXFLAGS += -arch $arch" >> "$mkfile"
+                echo "EXTRA_LFLAGS += -arch $arch" >> "$mkfile"
             done
             cat "$in_mkfile" >> "$mkfile"
             echo "-include \$(notdir \$(DEPEND_SRC:%.cpp=%.d))" >> "$mkfile"