x86: unify current_text_addr

current_text_addr() has a different implementation in x86_64 and
i386, but it is not fundamentally different. I stick to the i386
implementation, that seem to be a common base, and move it to processor.h

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>