Enable remote attestation by librats in SGX mode (#1445)

Add library librats, update SGX build scripts, add sample and update document.

samples/sgx-ra/wasm-app/CMakeLists.txt

@@ -0,0 +1,38 @@
+# Copyright (c) 2022 Intel Corporation
+# Copyright (c) 2020-2021 Alibaba Cloud
+# 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}/../../..)
+set (LIB_RATS_DIR ${WAMR_ROOT_DIR}/core/iwasm/libraries/lib-rats)
+
+set (CMAKE_C_LINK_FLAGS "")
+set (CMAKE_CXX_LINK_FLAGS "")
+if (APPLE)
+    set (HAVE_FLAG_SEARCH_PATHS_FIRST 0)
+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)
+set_target_properties(test.wasm PROPERTIES INCLUDE_DIRECTORIES ${LIB_RATS_DIR})
+target_link_libraries(test.wasm)