benchmark: forward execArgv to spawned childs

Modify the benchmark runner to forward `process.execArgv`
to child processes spawned to run individual scenarios (configs).

benchmark/common.js

@@ -131,6 +131,7 @@ Benchmark.prototype._run = function() {
     var argv = queue[i++];
     if (!argv)
       return;
+    argv = process.execArgv.concat(argv);
     var child = spawn(node, argv, { stdio: 'inherit' });
     child.on('close', function(code, signal) {
       if (code)