i8259: fix final ugliness

Introduce IRQx_VECTOR on 32-bit, so that #ifdef noise is kept
down. There should be no object code change.

[ mingo@elte.hu: merged to x86/irq not x86/i8259 due to x86/irq having
  restructured the vector code into asm-x86/irq_vectors.h, which this
  patch touches. ]

Signed-off-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Ingo Molnar <mingo@elte.hu>