Fix some chinese characters lookup error under Mac OS

The rev_index[] array should only contain entries where (ucs != ch).

For GBK encoding, the old code will keep 2 entries for each Chinese
character, one in which (ch = ucs), in the other (ch = gbk_encode(ucs)).

This way, when reversing (calling luitReverse), it is dependent on the
system's qsort() and bsearch() library API, that which entry will be
looked up.

For the Chinese character '会', on my Mac OS, the wrong entry (where
ch == ucs) will be looked up.