x86: rename the i8259_32/64.c leftovers to initirq_32/64.c

The leftovers of the i8259 unification have nothing to do with i8259
at all. They contain interrupt init code and the i8259_xx name is just
misleading now.

Rename them to initirq_32/64.c

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