samples/native-lib: Add an example to use wamr API from native lib (#1649)

Real world native libs likely need to access the wasm_runtime_xxx API.
This example demonstrates it.

Build vmlib as a shared lib to make it straightforward to share a
single runtime instance between iwasm and native libs.

samples/native-lib/test_hello2.c

@@ -0,0 +1,59 @@
+/*
+ * Copyright (C) 2019 Intel Corporation.  All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ */
+
+/*
+ * This example basically does the same thing as test_hello.c,
+ * using wasm_export.h API.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "wasm_export.h"
+
+static int
+test_hello2_wrapper(wasm_exec_env_t exec_env, uint32_t nameaddr,
+                    uint32_t resultaddr, uint32_t resultlen)
+{
+    /*
+     * Perform wasm_runtime_malloc to check if the runtime has been
+     * initialized as expected.
+     * This would fail with "memory hasn't been initialize" error
+     * unless we are not sharing a runtime with the loader app. (iwasm)
+     */
+    void *p = wasm_runtime_malloc(1);
+    if (p == NULL) {
+        return -1;
+    }
+    wasm_runtime_free(p);
+
+    wasm_module_inst_t inst = wasm_runtime_get_module_inst(exec_env);
+    if (!wasm_runtime_validate_app_str_addr(inst, nameaddr)
+        || !wasm_runtime_validate_app_addr(inst, resultaddr, resultlen)) {
+        return -1;
+    }
+    const char *name = wasm_runtime_addr_app_to_native(inst, nameaddr);
+    char *result = wasm_runtime_addr_app_to_native(inst, resultaddr);
+    return snprintf(result, resultlen,
+                    "Hello, %s. This is %s! Your wasm_module_inst_t is %p.\n",
+                    name, __func__, inst);
+}
+
+/* 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_hello2, "(iii)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);
+}