GH-111804: Drop posix.fallocate() under WASI (GH-111869)

Drop posix.fallocate() under WASI.

The underlying POSIX function, posix_fallocate(), was found to vary too
much between implementations to remain in WASI. As such, while it was
available in WASI preview1, it's been dropped in preview2.

Author: brettcannon

Misc/NEWS.d/next/Library/2023-11-08-15-58-57.gh-issue-111804.uAXTOL.rst

@@ -0,0 +1,2 @@
+Remove posix.fallocate() under WASI as the underlying posix_fallocate() is
+not available in WASI preview2.

Modules/clinic/posixmodule.c.h

@@ -11891,7 +11891,10 @@ os_truncate_impl(PyObject *module, path_t *path, Py_off_t length)
 #endif
 
 
-#if defined(HAVE_POSIX_FALLOCATE) && !defined(POSIX_FADVISE_AIX_BUG)
+/* GH-111804: Due to posix_fallocate() not having consistent semantics across
+   OSs, support was dropped in WASI preview2. */
+#if defined(HAVE_POSIX_FALLOCATE) && !defined(POSIX_FADVISE_AIX_BUG) && \
+    !defined(__wasi__)
 /*[clinic input]
 os.posix_fallocate
 
@@ -11929,7 +11932,7 @@ os_posix_fallocate_impl(PyObject *module, int fd, Py_off_t offset,
     errno = result;
     return posix_error();
 }
-#endif /* HAVE_POSIX_FALLOCATE) && !POSIX_FADVISE_AIX_BUG */
+#endif /* HAVE_POSIX_FALLOCATE) && !POSIX_FADVISE_AIX_BUG && !defined(__wasi__) */
 
 
 #if defined(HAVE_POSIX_FADVISE) && !defined(POSIX_FADVISE_AIX_BUG)