Add benchmarks (#979)

Add scripts to build and run benchmark of coremark, polybench,
sightglass and jetstream2. And add documents.

tests/benchmarks/jetstream/README.md

@@ -0,0 +1,29 @@
+# Introduction
+
+[JetStream 2](https://browserbench.org/JetStream) is a JavaScript and WebAssembly benchmark suite focused on the most advanced web applications. It rewards browsers that start up quickly, execute code quickly, and run smoothly.
+
+**Source**: https://browserbench.org/JetStream/in-depth.html
+
+# Building
+
+Please build iwasm and wamrc, refer to:
+- [Build iwasm on Linux](../../../doc/build_wamr.md#linux), or [Build iwasm on MacOS](../../../doc/build_wamr.md#macos)
+- [build wamrc AOT compiler](../../../README.md#build-wamrc-aot-compiler)
+
+And install emsdk, refer to [the guide](https://emscripten.org/docs/getting_started/downloads.html). Don't forget to activate
+ emsdk and set up environment variables. For example, use instructions below to install it under /opt and activate it:
+``` bash
+$ cd /opt
+$ git clone https://github.com/emscripten-core/emsdk.git
+$ cd emsdk
+$ git pull
+$ ./emsdk install latest
+$ ./emsdk activate latest
+$ echo "source /opt/emsdk/emsdk_env.sh" >> "${HOME}"/.bashrc
+```
+
+And then run `./build.sh` to build the source code, the folder `out` will be created and files will be generated under it.
+
+# Running
+
+Run `./run_aot.sh` to test the benchmark, the native mode and iwasm aot mode will be tested for each workload, and the file `report.txt` will be generated.