[CPT][IA64] save/restore NaT values
This patch closes two remaining holes in IA64 cpt implementation,
both are of no immediate practical value, but neraly impossible to fix
after we freeze layout of cpt image structs.
1. Migration between hosts with different layout of struct thread_info,
which is possible, if some new bits are added to thread_info in newer
2. NaT bits are acurately saved and restored. This is required only
when some application uses control speculative loads, current compilers
are not able to do this, but this can change.