liblink, cmd/gc: fix incompatible type signatures on Plan 9

R=ality, golang-dev, r, rsc
CC=golang-dev
https://golang.org/cl/39640043

include/link.h

@@ -43,6 +43,9 @@ typedef	struct	Library	Library;
 typedef	struct	Pcln	Pcln;
 typedef	struct	Pcdata	Pcdata;
 
+// prevent incompatible type signatures between liblink and 8l on Plan 9
+#pragma incomplete struct Node
+
 struct	Addr
 {
 	vlong	offset;
@@ -111,6 +114,9 @@ struct	Prog
 	char	mode;	/* 16, 32, or 64 */
 };
 
+// prevent incompatible type signatures between liblink and 8l on Plan 9
+#pragma incomplete struct Section
+
 struct	LSym
 {
 	char*	name;

src/cmd/gc/go.h

@@ -129,6 +129,9 @@ struct	Val
 	} u;
 };
 
+// prevent incompatible type signatures between libgc and 8g on Plan 9
+#pragma incomplete struct Array
+
 typedef	struct	Array	Array;
 typedef	struct	Bvec	Bvec;
 typedef	struct	Pkg Pkg;