Enable POWER support for Knative Serving

[gyliu513]

Expected Behavior

Knative Serving only support running on x86 but no POWER support yet, it would be great if Knative Serving can also run on POWER.

root@gyliu-c11:~# manifest-tool inspect gcr.io/knative-releases/github.com/knative/serving/cmd/activator@sha256:7e7abd88a292cf24a0db80d71fa16e8505dd288c241cab0b6716848f0e7fcc04
gcr.io/knative-releases/github.com/knative/serving/cmd/activator@sha256:7e7abd88a292cf24a0db80d71fa16e8505dd288c241cab0b6716848f0e7fcc04: manifest type: application/vnd.docker.distribution.manifest.v2+json
      Digest: sha256:7e7abd88a292cf24a0db80d71fa16e8505dd288c241cab0b6716848f0e7fcc04
Architecture: amd64
          OS: linux
    # Layers: 3
      layer 1: digest = sha256:8f125ded1b483229a567e48eece98cc8867c2bee55fb73e9cd65ceecc448b7c7
      layer 2: digest = sha256:d7409b568c4999bcc595709cc933f10f4c0c52b48a4ae8471a4201c41cff048c
      layer 3: digest = sha256:ee45d6e05d5b26e3702387cd7fe4925cfd54bd8981ecfbb60c06317a682c7425

/cc @zxDiscovery

[gyliu513]

/area build
/kind doc
/kind feature
/kind proposal

[mattmoor]

/assign @jonjohnsonjr

This needs fat image support in ko

[gyliu513]

@jonjohnsonjr @mattmoor do you think we need to enable POWER CI for this?

[jonjohnsonjr]

FWIW @seth-priya has been making progress on building knative for ppc here: knative/build#514

We can make this significantly easier, though.

Filed a bug to track some of that work here: https://github.com/google/go-containerregistry/issues/333

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