x86: provide tss_desc

Provide a new type, tss_desc, to represent the tss descriptor
in a unified way accross 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>