EBCDIC has the unicode bug too

EBCDIC has the unicode bug too

We have not had a working modern Perl on EBCDIC for some years.  When I
started out, comments and code led me to conclude erroneously that
natively it supported semantics for all 256 characters 0-255.  It turns
out that I was wrong; it natively (at least on some platforms) has the
same rules (essentially none) for the characters which don't correspond
to ASCII ones, as the rules for these on ASCII platforms.

A previous commit for 5.18 changed the docs about this issue.  This
current commit forces ASCII rules on EBCDIC platforms (even should there
be one that natively uses all 256).  To get all 256, the same things
like 'use feature "unicode_strings"' must now be done.