cmd/5c, cmd/5g, cmd/5l: fix array indexing warning under Clang 3.1

This should be the last of them.

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/6297043

src/cmd/5c/list.c

@@ -169,7 +169,7 @@ Dconv(Fmt *fp)
 
 	case D_SHIFT:
 		v = a->offset;
-		op = "<<>>->@>" + (((v>>5) & 3) << 1);
+		op = &"<<>>->@>"[(((v>>5) & 3) << 1)];
 		if(v & (1<<4))
 			sprint(str, "R%d%c%cR%d", v&15, op[0], op[1], (v>>8)&15);
 		else

src/cmd/5g/list.c

@@ -119,7 +119,7 @@ Dconv(Fmt *fp)
 
 	case D_SHIFT:
 		v = a->offset;
-		op = "<<>>->@>" + (((v>>5) & 3) << 1);
+		op = &"<<>>->@>"[(((v>>5) & 3) << 1)];
 		if(v & (1<<4))
 			sprint(str, "R%d%c%cR%d", v&15, op[0], op[1], (v>>8)&15);
 		else

src/cmd/5l/list.c

@@ -191,7 +191,7 @@ Dconv(Fmt *fp)
 
 	case D_SHIFT:
 		v = a->offset;
-		op = "<<>>->@>" + (((v>>5) & 3) << 1);
+		op = &"<<>>->@>"[(((v>>5) & 3) << 1)];
 		if(v & (1<<4))
 			snprint(str, sizeof str, "R%d%c%cR%d", v&15, op[0], op[1], (v>>8)&15);
 		else