Since memory and address spaces are a tile concept rather than a GT
concept, we need to plumb tile-based handling through lots of
memory-related code.
Note that one remaining shortcoming here that will need to be addressed
before media GT support can be re-enabled is that although the address
space is shared between a tile's GTs, each GT caches the PTEs
independently in their own TLB and thus TLB invalidation should be
handled at the GT level.