libroot/stdlib: Add qsort_r.

It's in POSIX-2024. Remove the libgnu implementation and just import
the changes from latest FreeBSD (dc36d6f9bb1753f3) instead.

While at it, put the non-standard sort functions behind
_DEFAULT_SOURCE.

headers/compatibility/gnu/stdlib.h

@@ -1,37 +0,0 @@
-/*
- * Copyright 2020 Haiku, Inc. All rights reserved.
- * Distributed under the terms of the MIT License.
- *
- * Authors:
- *		Isaac Turner, [email protected]
- *		Jacob Secunda, [email protected]
- */
-#ifndef _GNU_STDLIB_H_
-#define _GNU_STDLIB_H_
-
-
-#include_next <stdlib.h>
-#include <features.h>
-
-
-#ifdef _DEFAULT_SOURCE
-
-
-#ifdef __cplusplus
-extern "C" {
-#endif
-
-typedef int (*_compare_function_qsort_r)(const void*, const void*, void*);
-
-extern void qsort_r(void* base, size_t numElements, size_t sizeOfElement,
-	_compare_function_qsort_r, void* cookie);
-
-#ifdef __cplusplus
-}
-#endif
-
-
-#endif /* _DEFAULT_SOURCE */
-
-
-#endif /* _GNU_STDLIB_H_ */