Add definition to support alloc_align attribute.

gcc 4.9 added support for new alignment attribute alloc_align:

The alloc_align attribute is used to tell the compiler that the function
return value points to memory, where the returned pointer minimum
alignment is given by one of the functions parameters. GCC uses this
information to improve pointer alignment analysis.

This attribute is not (yet) available on clang.

sys/sys/cdefs.h

@@ -395,6 +395,12 @@
 #define	__alloc_size(x)
 #endif
 
+#if __has_attribute(alloc_align) || __GNUC_PREREQ__(4, 9)
+#define	__alloc_align(x)	__attribute__((__alloc_align__(x)))
+#else
+#define	__alloc_align(x)
+#endif
+
 /* XXX: should use `#if __STDC_VERSION__ < 199901'. */
 #if !__GNUC_PREREQ__(2, 7) && !defined(__INTEL_COMPILER)
 #define	__func__	NULL