Makefile: cscope may need to look in lib/$ARCH

When $ARCH != $TEST_DIR we should look there too. This patch cheats
though and makes cscope always look there, but then gets rid of the
duplicates generated when $ARCH == $TEST_DIR.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>