[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: NDavid Mosberger-Tang <dmosberger@gmail.com>
Signed-off-by: NTony Luck <tony.luck@intel.com>

arch/ia64/hp/sim/boot/boot_head.S

@@ -26,6 +26,7 @@ GLOBAL_ENTRY(_start)
 	movl sp = stack_mem+16384-16
 	bsw.1
 	br.call.sptk.many rp=start_bootloader
+0:	nop 0		  /* dummy nop to make unwinding work */
 END(_start)
 
 /*