[IA64] Save register stack contents on cpu start

The SN PROM uses the register stack in the slave loop. The contents
must be preserved for the OS to return to the slave loop via offlining
a cpu or for kexec. A 'flushrs" is needed to force the stack to be written
to memory prior to changing bspstore.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>