[IA64] Fix simulator boot (for real this time).

Thanks to Stephane, we've now worked out the real cause of the
`Linux  will not boot on simulator' problem.  Turns out it's a stack
overflow because the stack pointer wasn't being initialised properly
in boot_head.S (it was being initialised to the lowest instead of the
highest address of the stack, so the first push started to overwrite
data in the BSS).
Signed-off-by: NPeter Chubb <peterc@gelato.unsw.edu.au>
Signed-off-by: NTony Luck <tony.luck@intel.com>

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

@@ -22,7 +22,7 @@ GLOBAL_ENTRY(_start)
 	.save rp, r0
 	.body
 	movl gp = __gp
-	movl sp = stack_mem
+	movl sp = stack_mem+16384-16
 	bsw.1
 	br.call.sptk.many rp=start_bootloader
 END(_start)