Make generate_c also work if astnode does not have member

`instruction_set`
6 jobs for generalize-generate_c in 5 minutes and 54 seconds (queued for 36 seconds)
Status Job ID Name Coverage
  Test
passed #287664
cuda docker
build-documentation

00:00:52

passed #287663
cuda docker
flake8-lint

00:00:19

passed #287662
docker
minimal-conda

00:00:20

passed #287661
docker
minimal-ubuntu

00:00:17

passed #287660
win
minimal-windows

00:01:36

passed #287659
AVX cuda docker
tests-and-coverage

00:03:18