benchmark: pass execArgv to the benchmarking process

Benchmarker should pass exec flags (e.g. --no-crankshaft,
--turbofan-filter, --trace-opt etc) to the benchmarking process

Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
PR-URL: #928

benchmark/common.js

@@ -145,6 +145,7 @@ 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)