isr_tables: Simplify how the sw_irq_handler function is used

This is a migration from using code generation to using the C language
which we in the general case we should aways strive towards.

It is equivalent to the simplification that was done with
_irq_spurious here:
https://github.com/zephyrproject-rtos/zephyr/pull/7574



Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>