GN: don't run linux_find_llvm.py when is_system_compiler=true

- When passing is_system_compiler=true, stop setting implicitly
  the the cc/target_cc variables to $BUILD_CC/$CC and instead
  require that the embedder sets them explicitly.
- is_system_compiler=true doesn't run anymore linux_find_llvm.py.
  That is useless and triggers some check failures in the CrOS build.
- Update docs.
- Clean up some unused GN variables
- Clean up the use of underscore in some private GN variables.
  GN treats variables in .gni as non-exported only if they
  *start* with underscore.

Bug: 147789115 (#comment16)
Test: CI
Change-Id: I828e147c34b1b23c6d28f144c6102ddc4e82d25b
5 files changed
tree: 5a49b7754ae0ec8ce6e7a9de37182a589db60f6a
  1. bazel/
  2. build_overrides/
  3. buildtools/
  4. debian/
  5. docs/
  6. examples/
  7. gn/
  8. include/
  9. infra/
  10. protos/
  11. src/
  12. test/
  13. tools/
  14. ui/
  15. .clang-format
  16. .clang-tidy
  17. .gitignore
  18. .gn
  19. .style.yapf
  20. Android.bp
  21. Android.bp.extras
  22. BUILD
  23. BUILD.extras
  24. BUILD.gn
  25. CHANGELOG
  26. codereview.settings
  27. heapprofd.rc
  28. LICENSE
  29. METADATA
  30. MODULE_LICENSE_APACHE2
  31. OWNERS
  32. perfetto.rc
  33. PRESUBMIT.py
  34. README.chromium
  35. README.md
  36. TEST_MAPPING
  37. traced_perf.rc
  38. WORKSPACE
README.md

Perfetto - System profiling, app tracing and trace analysis

Perfetto is a production-grade open-source stack for performance instrumentation and trace analysis. It offers services and libraries and for recording system-level and app-level traces, native + java heap profiling, a library for analyzing traces using SQL and a web-based UI to visualize and explore multi-GB traces.

See https://perfetto.dev/docs or the /docs/ directory for documentation.