Enable register native with iwasm (#1120)

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

samples/native-lib/wasm-app/CMakeLists.txt

@@ -0,0 +1,35 @@
+# Copyright (C) 2019 Intel Corporation.  All rights reserved.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+cmake_minimum_required(VERSION 3.0)
+project(wasm-app)
+
+set (WAMR_ROOT_DIR ${CMAKE_CURRENT_SOURCE_DIR}/../../..)
+
+if (APPLE)
+    set (HAVE_FLAG_SEARCH_PATHS_FIRST 0)
+    set (CMAKE_C_LINK_FLAGS "")
+    set (CMAKE_CXX_LINK_FLAGS "")
+endif ()
+
+set (CMAKE_SYSTEM_PROCESSOR wasm32)
+set (CMAKE_SYSROOT                  ${WAMR_ROOT_DIR}/wamr-sdk/app/libc-builtin-sysroot)
+
+if (NOT DEFINED WASI_SDK_DIR)
+    set (WASI_SDK_DIR               "/opt/wasi-sdk")
+endif ()
+
+set (CMAKE_C_FLAGS                  "-nostdlib")
+set (CMAKE_C_COMPILER_TARGET        "wasm32")
+set (CMAKE_C_COMPILER               "${WASI_SDK_DIR}/bin/clang")
+
+set (CMAKE_EXE_LINKER_FLAGS
+    "-Wl,--max-memory=131072 -z stack-size=8192   \
+     -Wl,--no-entry,--strip-all                   \
+     -Wl,--export=__main_argc_argv                \
+     -Wl,--export=__heap_base,--export=__data_end \
+     -Wl,--allow-undefined"
+)
+
+add_executable(test.wasm main.c)
+target_link_libraries(test.wasm)

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;
+}