Dear Arch Linux (ARM) users,
You can now enjoy RetroArch directly from the official repositories, there’s even a dedicated group:
pacman -S libretro
A reasonable number of cores are already available in [community], don’t hesitate to request those that haven’t been added yet.
Note that you can also download cores (and all sorts of things) from the online updater included in the latest RetroArch, but you can’t mix downloaded cores with those from [community] as RetroArch will only search a single directory for cores.