6l, 8l: make string buffer big enough for 8 chars (and then some)

Fixes #221.

R=ken2
https://golang.org/cl/165086

src/cmd/6l/list.c

@@ -361,7 +361,7 @@ int
 Sconv(Fmt *fp)
 {
 	int i, c;
-	char str[30], *p, *a;
+	char str[100], *p, *a;
 
 	a = va_arg(fp->args, char*);
 	p = str;

src/cmd/8l/list.c

@@ -275,7 +275,7 @@ int
 Sconv(Fmt *fp)
 {
 	int i, c;
-	char str[30], *p, *a;
+	char str[100], *p, *a;
 
 	a = va_arg(fp->args, char*);
 	p = str;