unix_timestamp return in-consitent result for ambiguous timestamp value under different timezone

## Bug Report

Please answer these questions before submitting your issue. Thanks!
### 0. Background
In TiDB/MySQL, a timestamp column is timezone sensitive, and for some timezone, it has `Daylight Saving Time`. According to https://www.timeanddate.com/time/change/china/shanghai?year=1990 and https://www.timeanddate.com/time/change/chile/santiago?year=2022
for `Asia/Shanghai` timezone
> When local daylight time was about to reach
> Sunday, 16 September 1990, 02:00:00 clocks were turned backward 1 hour to
> Sunday, 16 September 1990, 01:00:00 local standard time instead.

for `America/Santiago` timezone
> When local daylight time is about to reach
> Sunday, 3 April 2022, 00:00:00 clocks are turned backward 1 hour to
> Saturday, 2 April 2022, 23:00:00 local standard time instead.

So `1990-09-15 16:00:00 UTC` and `1990-09-15 17:00:00 UTC` are ambiguous when in `Asia/Shanghai` timezone because both of them are `1990-09-16 01:00:00`. `2022-04-03 02:00:00 UTC` or `2022-04-03 03:00:00 UTC` are ambiguous when in `America/Santiago` because both of them are `2022-04-02 23:00:00`.

### 1. Minimal reproduce step (Required)

```
mysql> desc timestamp_table;
+-------+-----------+------+------+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-----------+------+------+---------+-------+
| id | int(11) | YES | | NULL | |
| value | timestamp | YES | | NULL | |
+-------+-----------+------+------+---------+-------+
2 rows in set (0.00 sec)

mysql> set time_zone='UTC';
Query OK, 0 rows affected (0.00 sec)

mysql> insert into timestamp_table values(1,'1990-09-15 15:59:59'),(2, '1990-09-15 16:00:00'),(3,'1990-09-15 17:00:00');
Query OK, 3 rows affected (0.01 sec)
Records: 3 Duplicates: 0 Warnings: 0

mysql> select unix_timestamp(value), value from timestamp_table;
+-----------------------+---------------------+
| unix_timestamp(value) | value |
+-----------------------+---------------------+
| 653414399 | 1990-09-15 15:59:59 |
| 653414400 | 1990-09-15 16:00:00 |
| 653418000 | 1990-09-15 17:00:00 |
+-----------------------+---------------------+
3 rows in set (0.01 sec)

mysql> set time_zone='Asia/Shanghai';
Query OK, 0 rows affected (0.00 sec)

mysql> select unix_timestamp(value), value from timestamp_table;
+-----------------------+---------------------+
| unix_timestamp(value) | value |
+-----------------------+---------------------+
| 653414399 | 1990-09-16 00:59:59 |
| 653418000 | 1990-09-16 01:00:00 |
| 653418000 | 1990-09-16 01:00:00 |
+-----------------------+---------------------+
3 rows in set (0.01 sec)
mysql> delete from timestamp_table;
Query OK, 3 rows affected (0.00 sec)

mysql> set time_zone='UTC';
Query OK, 0 rows affected (0.00 sec)

mysql> insert into timestamp_table values(1,'2022-04-03 01:59:59'),(2,'2022-04-03 02:00:00'),(3,'2022-04-03 03:00:00');
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0

mysql> select unix_timestamp(value), value from timestamp_table;
+-----------------------+---------------------+
| unix_timestamp(value) | value |
+-----------------------+---------------------+
| 1648951199 | 2022-04-03 01:59:59 |
| 1648951200 | 2022-04-03 02:00:00 |
| 1648954800 | 2022-04-03 03:00:00 |
+-----------------------+---------------------+
3 rows in set (0.01 sec)

mysql> set time_zone='America/Santiago';
Query OK, 0 rows affected (0.00 sec)

mysql> select unix_timestamp(value), value from timestamp_table;
+-----------------------+---------------------+
| unix_timestamp(value) | value |
+-----------------------+---------------------+
| 1648951199 | 2022-04-02 22:59:59 |
| 1648951200 | 2022-04-02 23:00:00 |
| 1648951200 | 2022-04-02 23:00:00 |
+-----------------------+---------------------+
3 rows in set (0.01 sec)
```
### 2. What did you expect to see? (Required)
I think a reasonable behavior should be one of either
- unix_timestamp return the same result for the same value under different timezone(This is the MySQL's behavior)
or at least
- the `rounding direct` for an ambiguous value should be the same, that is always return the greater or smaller result for all the ambiguous values
### 3. What did you see instead (Required)
for `1990-09-15 16:00:00 UTC` and `1990-09-15 17:00:00 UTC`
Under `UTC` timezone,
```
+-----------------------+---------------------+
| unix_timestamp(value) | value |
+-----------------------+---------------------+
| 653414399 | 1990-09-15 15:59:59 |
| 653414400 | 1990-09-15 16:00:00 |
| 653418000 | 1990-09-15 17:00:00 |
+-----------------------+---------------------+
```
Under `Asia/Shanghai` timezone,
```
+-----------------------+---------------------+
| unix_timestamp(value) | value |
+-----------------------+---------------------+
| 653414399 | 1990-09-16 00:59:59 |
| 653418000 | 1990-09-16 01:00:00 |
| 653418000 | 1990-09-16 01:00:00 |
+-----------------------+---------------------+
```
As we can see, `1990-09-15 16:00:00 UTC` and `1990-09-15 17:00:00 UTC` are ambiguous under `Asia/Shanghai` timezone, and TiDB return **greater** result for both of them under `Asia/Shanghai` timezone.

For `2022-04-03 02:00:00 UTC` or `2022-04-03 03:00:00 UTC`
Under `UTC` timezone
```
+-----------------------+---------------------+
| unix_timestamp(value) | value |
+-----------------------+---------------------+
| 1648951199 | 2022-04-03 01:59:59 |
| 1648951200 | 2022-04-03 02:00:00 |
| 1648954800 | 2022-04-03 03:00:00 |
+-----------------------+---------------------+
```
Under 'America/Santiago' timezone
```
+-----------------------+---------------------+
| unix_timestamp(value) | value |
+-----------------------+---------------------+
| 1648951199 | 2022-04-02 22:59:59 |
| 1648951200 | 2022-04-02 23:00:00 |
| 1648951200 | 2022-04-02 23:00:00 |
+-----------------------+---------------------+
```
As we can see, `2022-04-03 02:00:00 UTC` or `2022-04-03 03:00:00 UTC` are ambiguous under `America/Santiago` timezone, and TiDB return **smaller** result for both of them under `America/Santiago` timezone.

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

```
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version() |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v5.5.0-alpha-202-g077eb80f1
Edition: Community
Git Commit Hash: 077eb80f1317a54b1a73894d9f58cd6dfff19260
Git Branch: master
UTC Build Time: 2022-02-08 04:27:22
GoVersion: go1.17.1
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
```