include/sys/util: Fix WAIT_FOR for POSIX_ARCH

WAIT_FOR is a busy wait loop, which in the POSIX ARCH
should include a very minor delay in each iteration.

After this fix, tests/lib/sys_util does not need to
exclude native_posix anymore.

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