nodejs plugin support for ppc64el and s390x architectures

After adding nodejs plugin to the Hugo snap package, which used to build fine for all 6 supported architectures (i386, amd64, armhf, arm64, ppc64el, s390x) on Launchpad snap autobuilder, started failing on ppc64el and s390x with one of the following error messages:

    architecture not supported (ppc64el)

    architecture not supported (s390x)

See https://launchpadlibrarian.net/390345228/buildlog_snap_ubuntu_xenial_s390x_hugo_BUILDING.txt.gz for an example.

This error perplexed me initially because it was not obvious from the build log that the error it was coming from the nodejs plugin, and a quick look at https://packages.ubuntu.com/nodejs showed that nodejs does run on ppc64el and s390x on Ubuntu. I eventually found the "architecture not supported" string inside nodejs.py with the command:

    grep -r "architecture not supported" /snap/snapcraft/

Looking through Node.js's Previous Releases page at https://nodejs.org/en/download/releases/ , it turns out that "ppc64le" and "s390x" both appear in their regular releases starting in v6.6.0 (September 2016).

I think it should be a good time to add ppc64el and s390x to the list of supported architectures in nodejs.py. Link to GitHub pull request will be posted shortly.