x86-32: Export kernel_stack_pointer() for modules

Modules, in particular oprofile (and possibly other similar tools)
need kernel_stack_pointer(), so export it using EXPORT_SYMBOL_GPL().

Cc: Yang Wei <wei.yang@windriver.com>
Cc: Robert Richter <robert.richter@amd.com>
Cc: Jun Zhang <jun.zhang@intel.com>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/20120912135059.GZ8285@erda.amd.com


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>