How do you tag them?

And what is a Halal food shop?

For tagging, maybe you could use the tag for diet: osm.wiki/Key:diet
ie something like diet:halal=yes (if it sells some Halal food) or diet:halal=only (if it only sells Halal food).

Plus the usual tag for the shop type, eg shop=convenience, shop=deli, shop=supermarket etc.