I haven't worked with MariaDB. In my session for Elevate I used SQL Server Express, both for my local project and in AWS.
My initial impression is that you would use an ODBC driver for it ( such as https://mariadb.com/kb/en/about-mariadb-connector-odbc/ ) just as you would for MySQL (which MariaDB is derived from).
For what it's worth though, my experience with using MySQL/MariaDB has always been painful, particular if you use a storage engine that doesn't support transactions. I would opt for just about any of the other database options that AWS supports such as PostgreSQL, Oracle or SQL Server before going with MySQL or MariaDB. The Amazon Aurora database is MySQL/PostgreSQL compatible and I'd probably even look at that as well. That's just based on prior painful experience on my part, your mileage may vary.
Note that if you're looking at an AWS database to save licensing costs, only DynamoDB is free. And because that one is NoSQL, you'd have to get a third party ODBC driver (usually at some cost) in order to talk to it from PowerBuilder.