x86/apic: Move x2apic code to one place

Having several disjunct pieces of code for x2apic support makes
reading the code unnecessarily hard. Move it to one ifdeffed section.

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/20150115211702.445212133@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>