Switch arch migrator to cross-compilation by default

[jakirkham]

Given the ongoing issues we have been experiencing with Travis CI ( conda-forge/status#185 ), think we should start using cross-compilation on Azure as the default for new Arch Migrator PRs. This way we start with a configuration that usually works (sometimes with tweaks) while still building about as quickly as a native build

[jakirkham]

cc @beckermr (as we discussed this earlier)

[beckermr]

Thanks for the bump! This should be on for feedstocks for yaml migrators, but not versions. We have a separate r-base cross-compile mini-migrator that will need to be added in places.

[jakirkham]

Right was thinking about ArchRebuild.migrate. Is that the right place for this?

Regarding R, do you mean adding this to RUCRTCleanup.migrate? Or were you thinking of something else?

Also do migrators like these get combined when one is applied (like R)? Or do they both need to include this logic? Or is there some better way to handle this in one place that multiple migrators can use?

[beckermr]

We need to add them as minimigrators when making the main migrators.

[jakirkham]

If we want to convert existing feedstocks, sure

This issue was about changing the default in new migrations

[beckermr]

The defaults are set my mini migrators too for new things.