x86/apic: Implement single target IPI function for x2apic_cluster

x86/apic: Implement single target IPI function for x2apic_cluster

[ tglx: Split it out from the patch which provides the new callback ]

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mike Travis <travis@sgi.com>
Cc: Daniel J Blueman <daniel@numascale.com>
Link: http://lkml.kernel.org/r/20151104220848.817975597@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>