The `platform` field is pointless nesting: it's just stuff that happens
to be defined together, and that should be an implementation detail.
This instead makes `linux-kernel` and `gcc` top level fields in platform
configs. They join `rustc` there [all are optional], which was put there
and not in `platform` in anticipation of a change like this.
`linux-kernel.arch` in particular also becomes `linuxArch`, to match the
other `*Arch`es.
The next step after is this to combine the *specific* machines from
`lib.systems.platforms` with `lib.systems.examples`, keeping just the
"multiplatform" ones for defaulting.
Thanks to Repology / Alpine Linux I noticed that the GNS3 team continued
the development. From their README:
> This is a continuation of VPCS, based on the last development version
> and improved with patches wrote by various people from the community.
> The original VPCS code, which is unfortunately not maintained anymore,
> can be viewed on https://sourceforge.net/p/vpcs/code/
