Unfortunately the systemctl doesn’t take extra options the same way as pre-systemd init.d script did.
Although you can edit the /usr/lib/systemd/system/mysql.service and add the option to the ExecStart line, I would suggest to rather add it temporarily to your .cnf configuration file (for PXC 5.7 it will be /etc/percona-xtradb-cluster.conf.d/wsrep.cnf), just without the ‘–’ prefix.
Hope that works.