sample fuzzer: Move fuzzer specific code to sample and fix for native_sim

Move the LLVM fuzzing specific code out of the board main
file and into the sample.
That way we avoid needing to duplicate it for native_sim and
avoid having a very adhoc interface between the fuzzer test
and runner code.

Also ensure it works for native_sim and not just native_posix

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