doc: add teams for platform-specific issues

Specifies teams to cc for platform specific issues

 - Existing teams: @nodejs/platform-arm, @nodejs/platform-freebsd, and
@nodejs/platform-windows
 - New teams: @nodejs/platform-s390, @nodejs/platform-aix
 - Changed name: @nodejs/platform-solaris -> @nodejs/platform-smartos

PR-URL: https://github.com/nodejs/node/pull/9009
Reviewed-By: Julien Gilli <jgilli@nodejs.org>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

doc/onboarding-extras.md

@@ -29,6 +29,7 @@
 | upgrading c-ares | @jbergstroem |
 | upgrading http-parser | @jbergstroem, @nodejs/http |
 | upgrading libuv | @saghul |
+| platform specific | @nodejs/platform-{aix,arm,freebsd,macos,ppc,smartos,s390,windows} |
 
 
 When things need extra attention, are controversial, or `semver-major`: @nodejs/ctc