init_priorities: Fix for native_simulator based targets

Fix the init_prioties related cmake targets so they also
work with native_simulator based build targets.
Mostly this consists of pointing to the right file
(final build result instead of intermediate elf library)
and setting the dependency to that final build result.

Note that for the native_simulator based targets
the init priorities check can only be run on build
if we are building the final image (not just a partial
prelinked library),
and we are not assembling several images together
into one executable.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>