Arch: Generate locales

[cvlc12]

Fixes #45

• Solves 'undefined' language in Gnome settings (and probably others)
• Makes it easy to configure your own language with e.g. mkosi.local/mkosi.extra/etc/locale.gen:

$ cat mkosi.local/mkosi.extra/etc/locale.gen 
fr_FR.UTF-8 UTF-8
en_US.UTF-8 UTF-8

‣  Running postinstall script /home/particleos/mkosi.conf.d/arch/mkosi.postinst.chroot…
Generating locales...
  fr_FR.UTF-8... done
  en_US.UTF-8... done
Generation complete.