fix type error caused by recent change

R=gri
OCL=13545
CL=13545

src/syscall/errstr_darwin.go

@@ -336,11 +336,11 @@ func str(val int64) string {  // do it here rather than with fmt to avoid depend
 	var buf [32]byte;  // big enough for int64
 	i := len(buf)-1;
 	for val >= 10 {
-		buf[i] = val%10 + '0';
+		buf[i] = byte(val%10 + '0');
 		i--;
 		val /= 10;
 	}
-	buf[i] = val + '0';
+	buf[i] = byte(val + '0');
 	return string(buf)[i:len(buf)];
 }
 

src/syscall/errstr_linux.go

@@ -410,11 +410,11 @@ func str(val int64) string {  // do it here rather than with fmt to avoid depend
 	var buf [32]byte;  // big enough for int64
 	i := len(buf)-1;
 	for val >= 10 {
-		buf[i] = val%10 + '0';
+		buf[i] = byte(val%10 + '0');
 		i--;
 		val /= 10;
 	}
-	buf[i] = val + '0';
+	buf[i] = byte(val + '0');
 	return string(buf)[i:len(buf)];
 }