x/build: update ppc64le builders to gcc 9

[rsc]

#54275 happened because the builders are running gcc 8.3 and ran into a bug that is fixed in gcc 9.
@laboger suggests updating the builders to gcc 9.
Filing this to track that. I'm not sure who runs the builders exactly.

[pmur]

@rsc I've been effectively maintaining them for the last month or so working to modernize them. I created #54673 a few weeks ago for similar reasons.

I updated the container image on the power9 builder to run against an ubuntu 20.04 baseline. It gives us gcc-9 out of the box. If it behaves over the next week or so, I'll update the other ppc64le VM.