More comments about unimplemented SPRs.

Tag unused functions with unused attribute instead of using #ifdef (TODO)
  to ease tests: just have to enable the implementation in the cpu_defs table.


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@3306 c046a42c-6fe2-441c-8c8c-71466251a162