Introduce fixup_exception() on 64-bit and use it in kprobes to
eliminate an #ifdef.

Only 64-bit needs search_extable() due to a stepping bug.
Signed-off-by: NHarvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>