x/build: add LUCI linux-ppc64_power8 builder

[pmur]

This will be a builder for GOPPC64=power8 GOARCH=ppc64 GOOS=linux.

The hostname should be linux-ppc64-power8. The csr is attached: linux-ppc64-power8.csr.txt

[dmitshur]

Here's the certificate: linux-ppc64-power8-1712341779.cert.txt.

I'll also work on another CL needed for that hostname to be able to connect to the right pool in our LUCI instance soon. Edit: This is done (in internal CL crrev.com/i/7144177).

[gopherbot]

Change https://go.dev/cl/576935 mentions this issue: main.star: add linux-ppc64_power8 builder

[pmur]

It looks like it is up and running. Thank!