x86: introduce gate_desc type.

To account for the differences in gate descriptor in i386 and x86_64
a gate_desc type is introduced.

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>