regen/mk_invlists.pl: Simplify handling of early Unicode releases

regen/mk_invlists.pl: Simplify handling of early Unicode releases

Several Unicode properties are used internally in regex matching, for
example \b{wb} and \X.  The code is supposed to compile and work as best
it can on all Unicode releases.  But the rules for these have been
expanded by Unicode over the years, including new types of boundary
conditions.  Each such type is given an enum value in an enum type for
the given boundary.  For early Unicodes, this enum isn't present.

This commit simplifies how this generating code copes with this,
removing the hard-coded lists in favor of using a different mechanism
already in place for handling slightly different circumstances.