pw_presubmit: gn check system and generated files

- In presubmit, do gn check in a separate step and specify
  --check-generated and --check-system.
- Fix gn check issues in pw_fuzzer by adding a missing dependency.
- Disable include checking for generated pw_rpc headers since they
  include internal pw_rpc headers.

Change-Id: I8b9f03957ac60166eeb141bcba9dc493bfc6350c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/44562
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>