x86: unify asm nops

There's only one difference between the NOPs used in asm code for i386 and x86_64:
i386 has a lot more variants. The code is moved to processor.h, and adjusted
accordingly.

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>