Allow gcc to know result is aligned

Someone on the internet found out gcc only understand posix_memalign.

The alloc_align attribute may be applied to a function that returns
a pointer and takes at least one argument of an integer or enumerated
type. It indicates that the returned pointer is aligned on a boundary
given by the function argument at position.

Change-Id: I4b0af6ef3020da1fb460652117286193d5d72f1e
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4514
Reviewed-by: Fredrik Holmqvist <[email protected]>
Reviewed-by: Adrien Destugues <[email protected]>
Reviewed-by: waddlesplash <[email protected]>
Tested-by: Commit checker robot <[email protected]>

headers/posix/stdlib.h

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 Haiku, Inc. All Rights Reserved.
+ * Copyright 2002-2021 Haiku, Inc. All Rights Reserved.
  * Distributed under the terms of the MIT License.
  */
 #ifndef _STDLIB_H_
@@ -50,7 +50,7 @@ extern void		free(void *pointer);
 extern void		*malloc(size_t size);
 extern int		posix_memalign(void **_pointer, size_t alignment, size_t size);
 #ifdef _ISOC11_SOURCE
-extern void 	*aligned_alloc(size_t alignment, size_t size);
+extern void 	*aligned_alloc(size_t alignment, size_t size) _ALIGNED_BY_ARG(1);
 #endif
 extern void		*realloc(void *oldPointer, size_t newSize);