Enable register native with iwasm (#1120)

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

samples/native-lib/test_add.c

@@ -0,0 +1,32 @@
+/*
+ * Copyright (C) 2019 Intel Corporation.  All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "wasm_export.h"
+
+static int
+test_add_wrapper(wasm_exec_env_t *exec_env, int x, int y)
+{
+    return x + y;
+}
+
+/* clang-format off */
+#define REG_NATIVE_FUNC(func_name, signature) \
+    { #func_name, func_name##_wrapper, signature, NULL }
+
+static NativeSymbol native_symbols[] = {
+    REG_NATIVE_FUNC(test_add, "(ii)i")
+};
+/* clang-format on */
+
+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);
+}