arch-all pipelines

[bgermann]

It would be nice especially for the fetching phase if one could declare arch-all pipelines.
With the current implementation, a melange pipeline is run once on every architecture that the build is run for.
That means downloading the same files over and over again.
An obvious optimisation would be the following: Having a pipeline flag (i.e., arch-all) that declares it to be independent of the build architecture that is run preferrably in the native build environment.
If the melange pipeline starts with any number of such pipelines, they could be run once and hitting the first arch-dependent pipeline the builders can be spawned. If an arch-all pipeline is run after that, it can obviously also be executed within the builder.

build.arch is a variable that should be forbidden from being used in the proposed pipelines.