interrupts: safeguard isr_wrapper and isr_install

ld linker will only resolve undefined symbols inside functions that is
actually being called.

However, not all linkers behaves this way. Certain linkers, for example
armlink, resolves all undefined symbols even if during a later stage at
the linking the function will be pruned.

Therefore `ifdef CONFIG_GEN_ISR_TABLES` has been placed to safeguard
functions that will call undefined symbols when CONFIG_GEN_ISR_TABLES=y.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>