x86: unify system.h

This patch finishes the unification of system.h file.
i386 needs a constant to be defined, and it is defined inside an ifdef.

Other than that, pretty much nothing but includes are left in the arch
specific headers, and they are deleted.

[ mingo@elte.hu: 64-bit needs the cr8 access inlines. ]

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>