virtio-gpu without ARCH_HAS_SG_CHAIN

[Christoph Hellwig]

There is no good reason to not define ARCH_HAS_SG_CHAIN.  To fix
your immediate problem just select it from riscv, as riscv uses
the generic dma-direct code which is chained S/G safe by definition.

And if you want to get extra points do a quick audit of the remaning
iommu drivers on architectures that don't define it so that we can
kill the symbol.