Build binaries and container images for ppc64le.

[kishen-v]

Changes in this PR:

1. Add support to build binaries and images for ppc64le architecture.

2. Address warnings regarding labels in LegacyKeyValueFormat used in the image:

 - LegacyKeyValueFormat: "LABEL key=value" should be used instead of legacy "LABEL key value" format (line 7)
 - LegacyKeyValueFormat: "LABEL key=value" should be used instead of legacy "LABEL key value" format (line 8)

Fixed and validated through a build.

            "Labels": {
                "org.opencontainers.image.description": "Cloudflare's PKI toolkit",
                "org.opencontainers.image.source": "https://github.com/cloudflare/cfssl"
            }

[kishen-v]

Hi @nickysemenza,
Can you PTAL at the changes when time permits? I'm looking forward to having the artifacts built for ppc64le!

Thanks.