Support `_gbk` stmts

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
```mysql
select upper(_gbk '8Ca} ');
```
### 2. What did you expect to see? (Required)
```mysql
+---------------------+
| upper(_gbk '8Ca} ') |
+---------------------+
| 8CA} |
+---------------------+
1 row in set (0.00 sec)
```

### 3. What did you see instead (Required)

```mysql
ERROR 1115 (42000): Unsupported character introducer: 'gbk'
```

### 4. What is your TiDB version? (Required)