multimedia/arcan: use luajit-openresty on all architectures
lang/luajit hasn't been updated in years. Alpine, Debian, Fedora,
Gentoo, Guix, Mageia, Nix, Void updated to 2.1.0 beta3 or snapshot.
Of those Guix and Void have Arcan package. So, switch to 2.1 as well
for better stability and non-x86 support.