Build Failure in posixmodule.c with Clang, ThreadSanitizer, and free-threaded build enabled

Description:

A build failure occurs in Modules/posixmodule.c when compiling CPython from the main branch using Clang with ThreadSanitizer and the --disable-gil flag.

The compilation fails with two distinct types of errors:

The compiler reports that the statx struct has no member named stx_atomic_write_unit_max_opt. This error originates from the MM macro expansion within the pystatx_result_members definition.
Subsequently, multiple errors are reported for an invalid application of sizeof to an incomplete type PyMemberDef[]. This happens when the Py_ARRAY_LENGTH macro is used on pystatx_result_members, indicating that the array's definition was not completed due to the initial error.

This suggests that the feature detection or conditional compilation logic for the stx_atomic_write_unit_max_opt member is not functioning correctly under this specific build configuration.

Steps to Reproduce:

Check out the main branch of cpython.

Configure the build with the following command:

CC=clang CXX=clang++ ./configure --with-thread-sanitizer --with-pydebug --disable-gil

Run the build:
```
make
```

Expected Behavior:
The build process should complete successfully.

Actual Behavior:
The build fails with the following errors logged from Modules/posixmodule.c:

../Modules/posixmodule.c:3369:5: error: no member named 'stx_atomic_write_unit_max_opt' in 'statx'
3369 |     MM(stx_atomic_write_unit_max_opt, Py_T_UINT, atomic_write_unit_max_opt,
|     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3370 |         "maximum optimized size for direct I/O with torn-write protection"),
|         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./Modules/posixmodule.c:3325:20: note: expanded from macro 'MM'
3325 |     M(#attr, type, offsetof(Py_statx_result, stx.stx_##member), doc)
|     ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/llvm-20/lib/clang/20/include/__stddef_offsetof.h:16:24: note: expanded from macro 'offsetof'
16 | #define offsetof(t, d) __builtin_offsetof(t, d)
|                        ^
./Modules/posixmodule.c:3323:18: note: expanded from macro 'M'
3323 |     {attr, type, offset, Py_READONLY, PyDoc_STR(doc)}
|                  ^~~~~~
./Modules/posixmodule.c:3427:28: error: invalid application of 'sizeof' to an incomplete type 'PyMemberDef[]' (aka 'struct PyMemberDef[]')
3427 |     for (size_t i = 0; i < Py_ARRAY_LENGTH(pystatx_result_members) - 1; ++i) {
|                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./Include/pymacro.h:190:12: note: expanded from macro 'Py_ARRAY_LENGTH'
190 |     (sizeof(array) / sizeof((array)[0]))
|            ^~~~~~~
./Modules/posixmodule.c:3447:9: error: invalid application of 'sizeof' to an incomplete type 'PyMemberDef[]' (aka 'struct PyMemberDef[]')
3447 |     if (Py_ARRAY_LENGTH(pystatx_result_members) > 1
|         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./Include/pymacro.h:190:12: note: expanded from macro 'Py_ARRAY_LENGTH'
190 |     (sizeof(array) / sizeof((array)[0]))
|            ^~~~~~~

Environment:

Compiler: Clang
Configuration Flags: --with-thread-sanitizer --with-pydebug --disable-gil

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Build Failure in posixmodule.c with Clang, ThreadSanitizer, and free-threaded build enabled #140159

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Uh oh!

Build Failure in posixmodule.c with Clang, ThreadSanitizer, and free-threaded build enabled #140159

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