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/os/BeBuild.h

@@ -84,9 +84,11 @@
 	__attribute__((format(__printf__, _format_, _args_)))
 
 #if __GNUC__ >= 4
+# define _ALIGNED_BY_ARG(_no_) __attribute__((alloc_align(_no_)))
 # define _EXPORT __attribute__((visibility("default")))
 # define B_ALWAYS_INLINE __attribute__((always_inline)) inline
 #else
+# define _ALIGNED_BY_ARG(_no_)
 # define _EXPORT
 #endif
 #define _IMPORT