regex: Gnulib unibyte RRI uses bytes not chars

Adjust the non-glibc code to agree with what Gawk needs for
rational range interpretation (RRI) for regular expression ranges.
In unibyte locales, Gawk wants ranges to use the underlying byte
rather than the character code point.  This change does not affect
glibc proper.
* posix/regcomp.c (parse_byte) [!LIBC && RE_ENABLE_I18N]:
In unibyte locales, use the byte value rather than
running it through btowc.