x/build: add LUCI linux-ppc64le_power9 builder

[pmur]

This will be a builder for GOPPC64=power9 GOARCH=ppc64le GOOS=linux

The bot hostname is linux-ppc64le-power9. The csr is attached: linux-ppc64le-power9.csr.txt.

[dmitshur]

Thanks. Here's the certificate: linux-ppc64le-power9-1712180354.cert.txt.

[gopherbot]

Change https://go.dev/cl/576138 mentions this issue: main.star: add linux-ppc64le_power{9,10} builders

[pmur]

Builder should be up and running.