board native_sim/posix: Provide NEVER/NSI_NEVER definitions

Some native_posix specific apps/tests use the macro NEVER
provided it also for native_sim until all have switched
to the new NSI_NEVER.

And ensure NSI_NEVER is also avaiable when building
for native_posix to ease the transition.

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