rlibc: fix bug in `memcmp()`

src/librlibc/lib.rs

@@ -95,7 +95,7 @@
         let a = *offset(s1, i as int);
         let b = *offset(s2, i as int);
         if a != b {
-            return (a - b) as i32
+            return a as i32 - b as i32
         }
         i += 1;
     }