[IA64] Restore registers in the stack on INIT

Registers are not saved anywhere when INIT comes during fsys mode and
we cannot know what happened when we investigate vmcore captured by
kdump. This patch adds new function finish_pt_regs() so registers can
be saved in such a case.

Signed-off-by: Takao Indoh <indou.takao@jp.fujitsu.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>