As MariaDB tables are expected in a fixed order, the cross upgrade
previously placed roles, default_role and max_statement_time after
now unused mysql-5.7 columns.

Test:

$ cp -a /usr/local/mysql-5.7.31/data/ /tmp/m57data
$ sql/mysqld --no-defaults --skip-networking --datadir=/tmp/m57data  --socket=/tmp/${PWD##*/}.sock   --verbose --lc-messages-dir=$PWD/sql/share
2020-10-02 11:02:05 140135193212864 [Note] sql/mysqld (mysqld 10.2.34-MariaDB) starting as process 1457667 ...
2020-10-02 11:02:05 140135193212864 [Note] InnoDB: Mutexes and rw_locks use GCC atomic built

$ client/mysql_upgrade --no-defaults -u root -pbob  -S /tmp/build-mariadb-server-10.2.sock
MySQL upgrade detected
Phase 1/7: Checking and upgrading mysql database
Processing databases
mysql
mysql.columns_priv                                 OK
mysql.db                                           OK
mysql.engine_cost                                  OK
mysql.event                                        OK
mysql.func                                         OK
mysql.gtid_executed                                OK
mysql.help_category                                OK
mysql.help_keyword                                 OK
mysql.help_relation                                OK
mysql.help_topic                                   OK
mysql.innodb_index_stats                           OK
mysql.innodb_table_stats                           OK
mysql.ndb_binlog_index                             OK
mysql.plugin                                       OK
mysql.proc                                         OK
mysql.procs_priv                                   OK
mysql.proxies_priv                                 OK
mysql.server_cost                                  OK
mysql.servers                                      OK
mysql.slave_master_info                            OK
mysql.slave_relay_log_info                         OK
mysql.slave_worker_info                            OK
mysql.tables_priv                                  OK
mysql.time_zone                                    OK
mysql.time_zone_leap_second                        OK
mysql.time_zone_name                               OK
mysql.time_zone_transition                         OK
mysql.time_zone_transition_type                    OK
mysql.user                                         OK
Upgrading from a version before MariaDB-10.1
Phase 2/7: Installing used storage engines

Result:

| user  | CREATE TABLE `user` (
  `Host` char(60) COLLATE utf8_bin NOT NULL DEFAULT '',
  `User` char(80) COLLATE utf8_bin NOT NULL DEFAULT '',
  `Password` char(41) CHARACTER SET latin1 COLLATE latin1_bin NOT NULL DEFAULT '',
  `Select_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Insert_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Update_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Delete_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Create_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Drop_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Reload_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Shutdown_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Process_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `File_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Grant_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `References_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Index_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Alter_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Show_db_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Super_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Create_tmp_table_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Lock_tables_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Execute_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Repl_slave_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Repl_client_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Create_view_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Show_view_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Create_routine_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Alter_routine_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Create_user_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Event_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Trigger_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `Create_tablespace_priv` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `ssl_type` enum('','ANY','X509','SPECIFIED') CHARACTER SET utf8 NOT NULL DEFAULT '',
  `ssl_cipher` blob NOT NULL,
  `x509_issuer` blob NOT NULL,
  `x509_subject` blob NOT NULL,
  `max_questions` int(11) unsigned NOT NULL DEFAULT 0,
  `max_updates` int(11) unsigned NOT NULL DEFAULT 0,
  `max_connections` int(11) unsigned NOT NULL DEFAULT 0,
  `max_user_connections` int(11) NOT NULL DEFAULT 0,
  `plugin` char(64) CHARACTER SET latin1 NOT NULL DEFAULT '',
  `authentication_string` text COLLATE utf8_bin NOT NULL,
  `password_expired` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `is_role` enum('N','Y') CHARACTER SET utf8 NOT NULL DEFAULT 'N',
  `default_role` char(80) COLLATE utf8_bin NOT NULL DEFAULT '',
  `max_statement_time` decimal(12,6) NOT NULL DEFAULT 0.000000,
  `password_last_changed` timestamp NULL DEFAULT NULL,
  `password_lifetime` smallint(5) unsigned DEFAULT NULL,
  `account_locked` enum('N','Y') COLLATE utf8_bin NOT NULL DEFAULT 'N',
  PRIMARY KEY (`Host`,`User`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_bin COMMENT='Users and global privileges' |

MariaDB [(none)]>  CREATE ROLE `aRole`;
Query OK, 0 rows affected (0.00 sec)

MariaDB [(none)]> SET ROLE `aRole`;
Query OK, 0 rows affected (0.00 sec)

MariaDB [(none)]> FLUSH PRIVILEGES;
Query OK, 0 rows affected (0.00 sec)

MariaDB [(none)]> SET ROLE `aRole`;
Query OK, 0 rows affected (0.00 sec)

MariaDB [(none)]> SELECT `User`, `is_role` FROM `mysql`.`user`;
+---------------+---------+
| User          | is_role |
+---------------+---------+
| root          | N       |
| mysql.session | N       |
| mysql.sys     | N       |
| dan           | N       |
| aRole         | Y       |
+---------------+---------+
5 rows in set (0.00 sec)

Reviewer: Anel Husakovic