x86: remove magic number from ACPI sleep stack buffer

x86_64 SMP suspend to RAM uses a 10k temporary stack for saving the
kernel state, but only 4k of it is used. Shrink it to 4k.

Signed-off-by: Matt Mackall <mpm@selenic.com>
Acked-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Len Brown <len.brown@intel.com>