benchmark: allow easy passing of process flags

PR-URL: #28986
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

benchmark/common.js

@@ -25,6 +25,10 @@ function Benchmark(fn, configs, options) {
   if (options && options.flags) {
     this.flags = this.flags.concat(options.flags);
   }
+  if (process.env.NODE_BENCHMARK_FLAGS) {
+    const flags = process.env.NODE_BENCHMARK_FLAGS.split(/\s+/);
+    this.flags = this.flags.concat(flags);
+  }
   // Holds process.hrtime value
   this._time = [0, 0];
   // Used to make sure a benchmark only start a timer once