utf8.h: Correct improper EBCDIC conversion

utf8.h: Correct improper EBCDIC conversion

These macros were incorrect for EBCDIC.  The relationships are based on
I8, the intermediate-utf8 defined for UTF-EBCDIC, not the final encoding.
I was the culprit who did this orginally; I was confused by the names of
the conversion macros.  I'm adding names that are clearer to me; which
have already been defined in utfebcdic.h, but weren't defined for
non-EBCDIC platforms.