x86: Distangle ioapic and i8259

The proposed Moorestown support patches use an extra feature flag
mechanism to make the ioapic work w/o an i8259. There is a much
simpler solution.

Most i8259 specific functions are already called dependend on the irq
number less than NR_IRQS_LEGACY. Replacing that constant by a
read_mostly variable which can be set to 0 by the platform setup code
allows us to achieve the same without any special feature flags.

That trivial change allows us to proceed with MRST w/o doing a full
blown overhaul of the ioapic code which would delay MRST unduly.

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