cmd/5l: move offset2 into Adr.u0 union to save 4/8 bytes for Adr/Prog resp.

sizeof(Adr) from 24 bytes down to 20 bytes.
sizeof(Prog) from 84 bytes down to 76 bytes.

5l linking cmd/godoc statistics:
Before:
Maximum resident set size (kbytes): 106668
After:
Maximum resident set size (kbytes):  99412

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/7100059

src/cmd/5l/l.h

@@ -68,21 +68,24 @@ struct	Adr
 {
 	union
 	{
-		int32	u0offset;
+		struct {
+			int32	offset;
+			int32	offset2; // argsize
+		} u0off;
 		char*	u0sval;
 		Ieee	u0ieee;
 		char*	u0sbig;
 	} u0;
 	Sym*	sym;
 	Sym*	gotype;
-	int32	offset2; // argsize
 	char	type;
 	char	reg;
 	char	name;
 	char	class;
 };
 
-#define	offset	u0.u0offset
+#define	offset	u0.u0off.offset
+#define	offset2	u0.u0off.offset2
 #define	sval	u0.u0sval
 #define	scon	sval
 #define	ieee	u0.u0ieee