Enable register native with iwasm (#1120)

Enable register native with iwasm application, add sample and update document.

samples/native-lib/README.md

@@ -0,0 +1,59 @@
+# "native-lib" sample introduction
+
+This sample demonstrates how to write required interfaces in native library, build it into a shared library and register the shared library to iwasm.
+
+The native library should provide `get_native_lib` API for iwasm to return the native library info, including the module name, the native symbol list and the native symbol count, so that iwasm can use them to regiter the native library, for example:
+
+```C
+static int
+foo_wrapper(wasm_exec_env_t *exec_env, int x, int y)
+{
+    return x + y;
+}
+
+#define REG_NATIVE_FUNC(func_name, signature) \
+    { #func_name, func_name##_wrapper, signature, NULL }
+
+static NativeSymbol native_symbols[] = {
+    REG_NATIVE_FUNC(foo, "(ii)i")
+};
+
+uint32_t
+get_native_lib(char **p_module_name, NativeSymbol **p_native_symbols)
+{
+    *p_module_name = "env";
+    *p_native_symbols = native_symbols;
+    return sizeof(native_symbols) / sizeof(NativeSymbol);
+}
+```
+
+## Preparation
+
+Please install WASI SDK, download the [wasi-sdk release](https://github.com/CraneStation/wasi-sdk/releases) and extract the archive to default path `/opt/wasi-sdk`.
+
+## Build the sample
+
+```bash
+mkdir build
+cd build
+cmake ..
+make
+```
+
+`iwasm`, one wasm module `test.wasm` and two shared libraries `libtest_add.so`, `libtest_sqrt.so`
+will be generated.
+
+## Run workload
+
+```bash
+cd build
+./iwasm --native-lib=libtest_add.so --native-lib=libtest_sqrt.so wasm-app/test.wasm
+```
+
+The output is:
+
+```bash
+Hello World!
+10 + 20 = 30
+sqrt(10, 20) = 500
+```