setarch->CARCH mapping

Created by: deep-42-thought

It might be desirable to provide setarch with the same architecture for differing CARCHs. E.g. archlinux32 has "i686" and "pentium4" architectures, which differ in config files and CFLAGS and possibly also inside the PKGBUILDs (e.g. different patches might be necessary). Currently, we do this via a hard-coded mapping inside arch-nspawn: https://git.archlinux32.org/devtools32/commit/?id=45513b8260331f86418384c2b5df191cc62e2f6d but as discussed on irc, it might be desirable to export this into a separate config file inside /usr/share/devtools, which would enable archlinux and downstream distributions to add even more such pseudo-architectures to build packages optimized for more specific hardware. If that is something, that would be desired, I can look into how to convert our hack into such a solution and open a pull-request for it.