x86: move constants to desc_defs.h

this patch moves constant definitions regarding descriptor types
from desc_32.h to desc_defs.h. The change from defines to enum
to comply with previous versions in desc_defs.h

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>