[IA64] get back PT_IA_64_UNWIND program header

Explicitly put the unwind section into its own program-header.  This
used to be unnecessary (probably because binutils did it for us), but
with current binutils (e.g., v2.17.50.20070804) we won't get
the PT_IA_64_UNWIND header without this patch which will break
unwinding in a debugger and simulators such as Ski.

Signed-off-by: David Mosberger-Tang <dmosberger@gmail.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>