Move declarations of bsp_interrupt_get_affinity() and
bsp_interrupt_set_affinity() to <bsp/irq-generic.h>. Canonicalize the
<bsp/irq.h> includes.
Implement bsp_interrupt_get_affinity() and bsp_interrupt_set_affinity() only if
needed (usually RTEMS_SMP).
Provide stub implementations for i386 to fix build errors.