!fixup run bench against all strings

This fixes v8 over-optimizations against a single string by running all
five of our strings against the function.

benchmark/buffers/buffer-bytelength-utf8.js

@@ -17,10 +17,15 @@ function main(conf) {
   var n = conf.n | 0;
   var len = conf.len | 0;
 
+  // make a string with them all mixed together
   var str = buildStr(len);
 
   bench.start();
   for (var i = 0; i < n; i++) {
+    Buffer.byteLength(chars[0], 'utf8');
+    Buffer.byteLength(chars[1], 'utf8');
+    Buffer.byteLength(chars[2], 'utf8');
+    Buffer.byteLength(chars[3], 'utf8');
     Buffer.byteLength(str, 'utf8');
   }
   bench.end(n);