[IA64] make unwinder stop at last frame of the bootloader

Add a dummy nop at the end of _start() to maintain the invariant that
the return-pointer (rp) always point to the calling function.  This
makes unwinding stop at the last frame, as it should.

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