main.star: add linux-loong64 builder

For golang/go#65398.

Change-Id: Ideeea51070055f154b8720ee811dd43563b544ec
Reviewed-on: https://go-review.googlesource.com/c/build/+/561339
Reviewed-by: Michael Knyszek <[email protected]>
TryBot-Bypass: Michael Knyszek <[email protected]>

Author: dr2chase