cmd/5l: reorder some struct fields to reduce memory consumption

Valgrind Massif result when linking godoc:
On amd64:
                    old          new         -/+
mem_heap_B       185844612    175358047    -5.7%
mem_heap_extra_B    773404       773137    -0.0%

On 386/ARM:
                    old          new         -/+
mem_heap_B       141775701    131289941    -7.4%
mem_heap_extra_B    737011       736955    -0.0%

R=golang-dev, r, dave
CC=golang-dev
https://golang.org/cl/6655045

src/cmd/5l/l.h

@@ -74,13 +74,12 @@ struct	Adr
 		char*	u0sbig;
 	} u0;
 	Sym*	sym;
+	Sym*	gotype;
+	int32	offset2; // argsize
 	char	type;
-	uchar	index; // not used on arm, required by ld/go.c
 	char	reg;
 	char	name;
-	int32	offset2; // argsize
 	char	class;
-	Sym*	gotype;
 };
 
 #define	offset	u0.u0offset