isr_tables: Simplify how the spurious irq function address is found

Instead of finding the address of the spurious irq function in the
intList section we now rely on the linker to find the address in the
_irq_spurious symbol.

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

In this specific case it makes the generated code 'irq_tables.c'
easier to read as we replace magic numbers with the &_irq_spurious
token.

Also, the path through the build system that _irq_spurious makes is
much shorter, so it is much easier for a user to understand how it is
used.

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