Change meaning of UNI_IS_INVARIANT on EBCDIC platforms

Change meaning of UNI_IS_INVARIANT on EBCDIC platforms

This should make more CPAN and other code work without change.  Usually,
unwittingly, code that says UNI_IS_INVARIANT means to use the native
platform code values for code points below 256, so acquiesce to the
expected meaning and make the macro correspond.  Since the native values
on ASCII machines are the same as Unicode, this change doesn't affect
code running on them.

A new macro, OFFUNI_IS_INVARIANT, is created for those few places that
really do want a Unicode value.  There are just a few places in the Perl
core like that, which this commit changes.