benchmark: Support passing v8 flags to benchmarks

The better to test --use-strict effects on performance.

(Spoiler: it has no measurable effect on performance.)

benchmark/common.js

@@ -18,6 +18,7 @@ if (module === require.main) {
   var spawn = require('child_process').spawn;
 
   runBenchmarks();
+}
 
 function runBenchmarks() {
   var test = tests.shift();
@@ -30,7 +31,9 @@ if (module === require.main) {
   console.error(type + '/' + test);
   test = path.resolve(dir, test);
 
-    var child = spawn(process.execPath, [ test ], { stdio: 'inherit' });
+  var a = process.execArgv || [];
+  a.push(test);
+  var child = spawn(process.execPath, a, { stdio: 'inherit' });
   child.on('close', function(code) {
     if (code)
       process.exit(code);
@@ -40,7 +43,6 @@ if (module === require.main) {
     }
   });
 }
-}
 
 exports.createBenchmark = function(fn, options) {
   return new Benchmark(fn, options);