x86: unify thread struct.

The thread_struct is not fundamentally different between architectures, and
this patch puts it in the common header. What's really unique for each of
them is enclosed in ifdefs.

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>