- Unbreak booting sparc64 kernels after the metadata unification in

- Unbreak booting sparc64 kernels after the metadata unification in
  r329190; sparc64 kernels are always 64-bit but with that revision
  in place, the loader was treating them as 32-bit ones.
- In order to reduce the likelihood of this kind of breakage in the
  future, #ifdef out md_load() on sparc64 and make md_load_dual() -
  which is currently local to metadata.c anyway - static.
- Make md_getboothowto() - also local to metadata.c - static.
- Get rid of the unused DTB pointer on sparc64.