Upgrade GA CI Linux VM's to Ubuntu 18.04 for rseq testing and other features

We're using the old Ubuntu 16.04 in our GA CI suite and it's too old to support rseq (4.18 is needed). Xref https://github.com/DynamoRIO/dynamorio/pull/4929#issuecomment-864151571, https://github.com/DynamoRIO/dynamorio/pull/4929#issuecomment-864160943

This issue covers upgrading to at least 18.04. IIRC we tried in the past and hit flaky tests and other problems? We may hit things like #4830.