x86: unify common parts of segment.h

Although segment handling in i386 and x86_64 are very different,
there's a common part. Put them in segment.h instead of arch specific
headers

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>