Add benchmarks (#979)

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

tests/benchmarks/coremark/run.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+
+# Copyright (C) 2019 Intel Corporation.  All rights reserved.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+IWASM="../../../product-mini/platforms/linux/build/iwasm"
+WAMRC="../../../wamr-compiler/build/wamrc"
+
+echo "Run coremark with native .."
+./coremark.exe
+
+echo "Run coremark with iwasm mode .."
+${IWASM} coremark.aot
+
+echo "Run coremakr with iwasm interpreter .."
+${IWASM} coremark.wasm