x86: irq_32 move 4kstacks code to one place

Move the 4KSTACKS related code to one place. This allows to un#ifdef
do_IRQ() and share the executed on stack for the stack overflow printk
and the softirq call.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>