gh-110014: Fix _POSIX_THREADS and _POSIX_SEMAPHORES usage (#110139)

* pycore_pythread.h is now the central place to make sure that
  _POSIX_THREADS and _POSIX_SEMAPHORES macros are defined if
  available.
* Make sure that pycore_pythread.h is included when _POSIX_THREADS
  and _POSIX_SEMAPHORES macros are tested.
* PY_TIMEOUT_MAX is now defined as a constant, since its value
  depends on _POSIX_THREADS, instead of being defined as a macro.
* Prevent integer overflow in the preprocessor when computing
  PY_TIMEOUT_MAX_VALUE on Windows:
  replace "0xFFFFFFFELL * 1000 < LLONG_MAX"
  with "0xFFFFFFFELL < LLONG_MAX / 1000".
* Document the change and give hints how to fix affected code.
* Add an exception for PY_TIMEOUT_MAX  name to smelly.py
* Add PY_TIMEOUT_MAX to the stable ABI