Wall spell be unsupported, e.g. to make an archway ?
Kind of, based on the provisions of the 2.5 dimensions of D&D.
Therefore, the original rules presume a 2d landscape. Walls, however, extend for a length in the vertical dimension as a function of their spell. There is no restriction that the contiguous squares be the bottom-most square, therefore there is no rule against it.
There is also no rule for it, as the wording deals only with two (and a half) dimensions. As the verticality of the wall is not a function of its dimensionality, but rather its power wording... this is one of those "ask your GM" grey areas that isn't much fun at all in this game.