cmd/gc: limit data disassembly to -SS

This makes -S useful again.

R=ken2
CC=golang-dev
https://golang.org/cl/6302054

src/cmd/5g/gobj.c

@@ -198,7 +198,8 @@ dumpfuncs(void)
 		if(isblank(pl->name))
 			continue;
 
-		if(debug['S']) {
+		// -S prints code; -SS prints code and data
+		if(debug['S'] && (pl->name || debug['S']>1)) {
 			s = S;
 			if(pl->name != N)
 				s = pl->name->sym;

src/cmd/6g/gobj.c

@@ -244,7 +244,8 @@ dumpfuncs(void)
 		if(isblank(pl->name))
 			continue;
 
-		if(debug['S']) {
+		// -S prints code; -SS prints code and data
+		if(debug['S'] && (pl->name || debug['S']>1)) {
 			s = S;
 			if(pl->name != N)
 				s = pl->name->sym;

src/cmd/8g/gobj.c

@@ -242,7 +242,8 @@ dumpfuncs(void)
 		if(isblank(pl->name))
 			continue;
 
-		if(debug['S']) {
+		// -S prints code; -SS prints code and data
+		if(debug['S'] && (pl->name || debug['S']>1)) {
 			s = S;
 			if(pl->name != N)
 				s = pl->name->sym;

src/cmd/gc/doc.go

@@ -47,7 +47,9 @@ Flags:
 	-N
 		disable optimizations
 	-S
-		write assembly language text to standard output
+		write assembly language text to standard output (code only)
+	-SS
+		write assembly language text to standard output (code and data)
 	-u
 		disallow importing packages not marked as safe
 	-V