x/build: add LUCI aix-ppc64 builder

[dmitshur]

There currently isn't a LUCI builder that tests the aix/ppc64 port (other than the misc-compile builder, which tests only that the port compiles). This is the tracking issue for it.

The next steps that a builder owner will need to follow to make progress here are documented https://go.dev/wiki/DashboardBuilders#luci-builders.

[laboger]

@ayappanec

[ayappanec]

We are working on getting a AIX machine in OSU lab for this purpose.

[dmitshur]

CC @golang/aix.

[pmur]

I'll try poking around with this since I came into access to the VM.

I've chosen the hostname as aix-ppc64-power8, cert attached: aix-ppc64-power8.csr.txt

(edit: fix hostname to match prevailing conventions, edit 2: revert to original naming)

[gopherbot]

Change https://go.dev/cl/600995 mentions this issue: main.star: mark aix-ppc64 as fixed capacity