treecompose-post.sh: keep all supported LANGs
In https://bugzilla.redhat.com/show_bug.cgi?id=1186757, it was
acknowledged that a just middle between deleting all the locales and
keeping them all was to only keep "supported" locales. There doesn't
seem to be such a list for Fedora, though RHEL does have one. So for
now, we piggyback on the RHEL definition as to what "supported" means.
As of right now, this increases the on-disk size of an F25 tree by 17M.
Its compressed size should be even more insignificant.