sparc64: fix pstate privilege bits

- refactor code to handle hpstate only if available for current cpu
- conditionally set hypervisor bit in hpstate register
- reorder softmmu indices so user accessable ones go first, translation context
  macros supervisor() and hypervisor() adjusted as well
- disable sparcv8 registers for TARGET_SPARC64
- fix cpu_mmu_index to use sparcv9 bits only

Signed-off-by: Igor V. Kovalenko <igor.v.kovalenko@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>