regen/regcharclass.pl: Add const cast

regen/regcharclass.pl: Add const cast

This is a follow-up to commit 9f2eed981068e7abbcc52267863529bc59e9c8c0,
which manually added const qualifiers to some generated code in order to
avoid some compiler warnings.  This changes the code generator to use
the same 'const' qualifier generally.  The code changed by the other
commit had been hand-edited after being generated to add branch
prediction, which would be too hard to program in at this time, so the
const additions also had to be hand-edited in.