MFC: r333955

• Unbreak booting sparc64 kernels after the metadata unification in r329190 (MFCed to stable/11 in r332150); 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.

Approved by: re (kib)