x86/ioapic: Provide stub functions for IOAPIC%3Dn

To avoid lots of ifdeffery provide proper stubs for setup_IO_APIC(),
enable_IO_APIC() and setup_ioapic_dest().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Tony Luck <tony.luck@intel.com>
Link: http://lkml.kernel.org/r/20150115211703.397170414@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>