x86: introduce ldt_desc type.

this patch introduces ldt_desc type to account for the differences
in the ldt descriptor in x86_64 and i386

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>