x86/smp: Move smp_function_call implementations into IPI code

Move it where it belongs. That allows to keep all the shorthand logic in
one place.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20190722105220.677835995@linutronix.de