x86: unify current.h

Simply stitch these together. There are just two definitions that are shared
but the file is resonably small and putting these things together shows that
further unifications requires a unification of the per cpu / pda handling
between both arches.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>