gh-108220: Internal header files require Py_BUILD_CORE to be defined (#108221)

* pycore_intrinsics.h does nothing if included twice
  (add #ifndef and #define).
* Update Tools/cases_generator/generate_cases.py to generate the
  Py_BUILD_CORE test.
* _bz2, _lzma, _opcode and zlib extensions now define the
  Py_BUILD_CORE_MODULE macro to use internal headers
  (pycore_code.h, pycore_intrinsics.h and pycore_blocks_output_buffer.h).