Arch Linux support

[kpcyrd]

This is the tracking issue for Arch Linux support. The list of current issues:

• only one worker may run at a time or they conflict with each other (Running repro in parallel archlinux/archlinux-repro#68)
• only one core is used to build (repro only uses one core to build archlinux/archlinux-repro#67)

[kpcyrd]

I guess we can consider this done, it has soon been around and stable for years at this point.