[libc] Remove legacy SYS_socketcall fallbacks (#197189)

This patch removes legacy SYS_socketcall fallback paths from all Linux
socket entry points. The individual syscall entry points were
[added](https://github.com/torvalds/linux/commit/9dea5dc921b5f4045a18c63eb92e84dc274d17eb)
in linux 4.3 (on x86, other architectures have had them even sooner).
Our policy is to support the lowest kernel version on
https://kernel.org/, which is 5.10 as of this writing.

This is motivated by the problems in

[testing](https://github.com/llvm/llvm-project/pull/196903#pullrequestreview-4263553670)
the fallback paths -- to make sure this even builds, one needs to get a
hold of very old kernel headers, or otherwise hack its build to force it
to select the fallback path.

New ABIs don't have the accept syscall (only accept4), so I've added an
accept->accept4 fallback.

Assisted by Gemini.
16 files changed
tree: fdd3ce93f8009d79dd809bdaf64503897e1fe361
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. flang-rt/
  11. libc/
  12. libclc/
  13. libcxx/
  14. libcxxabi/
  15. libsycl/
  16. libunwind/
  17. lld/
  18. lldb/
  19. llvm/
  20. llvm-libgcc/
  21. mlir/
  22. offload/
  23. openmp/
  24. orc-rt/
  25. polly/
  26. runtimes/
  27. third-party/
  28. utils/
  29. .clang-format
  30. .clang-format-ignore
  31. .clang-tidy
  32. .git-blame-ignore-revs
  33. .gitattributes
  34. .gitignore
  35. .mailmap
  36. CODE_OF_CONDUCT.md
  37. CONTRIBUTING.md
  38. LICENSE.TXT
  39. pyproject.toml
  40. README.md
  41. SECURITY.md
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