ACPI processor: Use safe_halt() rather than halt() in acpi_idle_play_dead()

ACPI code is shared by arch/x86 and arch/ia64. ia64 doesn't provide a plain
"halt()" function.  Use safe_halt() instead.

Signed-off-by: Tony Luck <tony.luck@intel.com>
Tested-by: Boris Ostrovsky <boris.ostrovsky@amd.com>
Signed-off-by: Len Brown <len.brown@intel.com>