cut down size of some libc struct members

these are all flags that can be single-byte values.

src/internal/libc.h

@@ -18,9 +18,9 @@ struct tls_module {
 };
 
 struct __libc {
-	int can_do_threads;
+	char can_do_threads;
-	int threaded;
+	char threaded;
-	int secure;
+	char secure;
 	int threads_minus_1;
 	size_t *auxv;
 	struct tls_module *tls_head;