Enable register native with iwasm (#1120)

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

samples/native-lib/wasm-app/main.c

@@ -0,0 +1,29 @@
+/*
+ * Copyright (C) 2019 Intel Corporation.  All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+int
+test_add(int x, int y);
+
+int
+test_sqrt(int x, int y);
+
+int
+main(int argc, char **argv)
+{
+    int x = 10, y = 20, res;
+
+    printf("Hello World!\n");
+
+    res = test_add(x, y);
+    printf("%d + %d = %d\n", x, y, res);
+
+    res = test_sqrt(x, y);
+    printf("sqrt(%d, %d) = %d\n", x, y, res);
+
+    return 0;
+}