util: Added GCN-GPU to cloudbuild_create_images

util: Added GCN-GPU to cloudbuild_create_images

This adds the GCN-GPU Docker image to this cloudbuild yaml file. We
use this file to create our Docker images:

```
gcloud builds submit --config \
util/cloudbuild/cloudbuild_create_images.yaml
```

The GCN-GPU docker image can be obtained at:
https://gcr.io/gem5-test/gcn-gpu

Change-Id: I7564db78a9f00507f5acc6cf6098de35f98b6fb1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33595
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>