dist/setup-config's header should mention $arch and $os

[23Skidoo]

dist/setup-config has the following header:

Saved package config for issue1244-1.0 written by Cabal-1.17.0 using ghc-7.4

This information is used for checking whether we should reconfigure. Now that we support cross-compilation, we should probably also save $arch and $os there.

[ttuegel]

I rather wish I had seen this before we changed the persistent build configuration's format. :/