sparc64: fix mmu context at trap levels above zero

- cpu_mmu_index return MMU_NUCLEUS_IDX if trap level is not zero
- cpu_get_tb_cpu_state: store trap level and primary context in flags
  this allows to restart code translation when address translation is changed
- stop translation block after writing to pstate and tl registers
- stop translation block after writing to alternate space
  this can be optimized to stop only if address translation can be changed
  by write operation (e.g. by comparing with MMU ASI values)

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