[IA64] unwind did not work for processes born with CLONE_STOPPED

[IA64] unwind did not work for processes born with CLONE_STOPPED

Minor problem for mainstream. Big problem for checkpoint/restore,
because all the stopped/traced processes are born in this state,
hence they cannot be checkpointed again due to failing unwind.

The problem was identified as assumption in kernel unwind library
that top level frame is different of syscall frame. It is the case
unless process was born with CLONE_STOPPED.

Author: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Signed-Off-By: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Signed-Off-By: Kirill Korotaev <dev@sw.ru>
Signed-off-by: Tony Luck <tony.luck@intel.com>