devel/binutils: Add GNU to MASTER_SITES

GNU mirrors also hosts binutils and as there are a lot more of those
set GNU as primary alias and fallback to SOURCEWARE

Reference:
https://lists.gnu.org/archive/html/info-gnu/2023-07/msg00009.html

Approved by:	portmgr (blanket)

devel/binutils/Makefile

@@ -3,7 +3,8 @@ DISTVERSION=	2.40
 PORTREVISION=	4
 PORTEPOCH?=	1
 CATEGORIES?=	devel
-MASTER_SITES=	SOURCEWARE/binutils/releases
+MASTER_SITES=	GNU \
+		SOURCEWARE/binutils/releases
 
 MAINTAINER?=	cy@FreeBSD.org
 COMMENT?=	GNU binary tools