x86_64: Remove serialize_cpu() inline

- It was redundant with sync_core()
- It was unused
- It was broken: no input arguments to cpuid; could fault randomly
  depending on eax contents.

Now it's gone.

[ tglx: arch/x86 adaptation ]

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>