Add IBMers to nodejs/platform-s390 #1099

IBM has a couple of people starting to look at Node.js, primarily for Linux on s390x.

• @abhayagarwal-dev
• @giritrivedi

They're starting out by looking at the citgm skips on that platform, e.g. nodejs/citgm#1140, nodejs/citgm#1138.

I would like to add them to the @nodejs/platform-s390 GitHub team so they can get notified when the team is mentioned and also be able to see the Jenkins CI results.