main.star: add plan9-{386, amd64, arm} builders

For golang/go#63599
For golang/go#63600
For golang/go#63601

Change-Id: I804b903615861667cc88c70b25296871788ef5ed
Reviewed-on: https://go-review.googlesource.com/c/build/+/536196
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Bypass: Carlos Amedee <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Carlos Amedee <[email protected]>

Author: cagedmantis