[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: Peter Chubb <peterc@gelato.unsw.edu.au>
Signed-off-by: Tony Luck <tony.luck@intel.com>