broken hueristic to determine memory vs. i/o (one should never make
an assumption that the bus_space_tag_t is a pointer, as this code
did).
- Fix the "can't map registers" error message.
- Garbage-collect some code that is not relevant to the GEM (which
was already #if 0'd out).
- Cluster all the SPARC-specific code into one place (will be
replaced with Properties once that is fleshed out).