percpu: use a kconfig variable to signal arch specific percpu setup

The use of the __GENERIC_PERCPU is a bit problematic since arches
may want to run their own percpu setup while using the generic
percpu definitions. Replace it through a kconfig variable.

Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Mike Travis <travis@sgi.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>