support GBK for builtin function convert

[xiongjiwei]

support convert other charsets to gbk. e.g. select convert('a' using gbk)
if the convert meets invalid gbk character use ? instead. e.g.

mysql> select convert(_utf8mb4 'abcㅂdd' using gbk);
+----------------------------------------+
| convert(_utf8mb4 'abcㅂdd' using gbk)  |
+----------------------------------------+
| abc?dd                                 |
+----------------------------------------+
1 row in set (0.00 sec)