You won't be able to install debian .deb files on Arch as it uses pacman to manage packages instead of aptitude. You can just pacman -S ghc
to install the precompiled binary for armv5 (it's in extra/), so there's no need to cross-compile it.
Of course if you wanted to specifically use a version compiled for armv6, you would have to install the armv5 one first to be able to compile another version.