AWS RDS MariaDB replication to GCP Cloud SQL or Azure SQL DB
I don't think it can be done solely in RDS, at least not directly between 2 DBaaS platforms.  Both vendors would need interoperability in their APIs.  I think your stuck standing up a Linux/Windows server in AWS/GCP/Azure to be a "on-prem" host to work in between the DBaaS platforms.  

This may be helpful: http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/MySQL.Procedural.Exporting.NonRDSRepl.html

Of course...


  Replication to an instance of MySQL running external to Amazon RDS is only supported during the time it takes to export a database from a MySQL DB instance. The replication should be terminated when the data has been exported and applications can start accessing the external instance.


Hopefully you need to do this for a migration, and not ongoing replication, or you don't care about being in a "supported" configuration.