Continuation of issue 221 fix. When 8g or 6g or 5g are called with a

UTF-8 string, Yconv() converts it into an octal sequence. If the
string converted to more than 30 bytes, the str buffer would
overflow. For example, 4 Greek runes became 32 bytes, 3 Hiragana
runes became 36 bytes, and 2 Gothic runes became 32 bytes. In
8l, 6l and 5l the function is Sconv(). For some reason, only 5l uses
the constant STRINGSZ (defined as 200) for the buffer size.

R=rsc
https://golang.org/cl/168045

src/cmd/5g/list.c

@@ -233,7 +233,7 @@ int
 Yconv(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/6c/list.c

@@ -353,7 +353,7 @@ 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/6g/list.c

@@ -313,7 +313,7 @@ int
 Yconv(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/8c/list.c

@@ -274,7 +274,7 @@ 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/8g/list.c

@@ -256,7 +256,7 @@ int
 Yconv(Fmt *fp)
 {
 	int i, c;
-	char str[30], *p, *a;
+	char str[100], *p, *a;
 
 	a = va_arg(fp->args, char*);
 	p = str;