Fix sgx porting issues: incorrect compile flags, porting func impl, document, etc. (#145)

* Fix sgx porting issues: incorrect compilation flags, porting function impl, document, etc.

* Update bh_platform.c

Add check for function bh_vprintf_sgx: check whether print_function is not NULL.

core/iwasm/products/linux-sgx/CMakeLists.txt

@@ -16,6 +16,7 @@ add_definitions(-DOPS_INPUT_OUTPUT=1)
 add_definitions(-DOPS_UNSAFE_BUFFERS=0)
 add_definitions(-DWASM_ENABLE_LOG=0)
 add_definitions(-Dbh_printf=bh_printf_sgx)
+add_definitions(-Dvprintf=bh_vprintf_sgx)
 
 if (NOT ("$ENV{VALGRIND}" STREQUAL "YES"))
   add_definitions(-DNVALGRIND)
@@ -78,13 +79,17 @@ endif ()
 set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -Wl,--gc-sections")
 set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -ffunction-sections -fdata-sections -Wall -Wno-unused-parameter -Wno-pedantic")
 
+set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -nostdinc -fvisibility=hidden -fpie -ffunction-sections -fdata-sections")
+
 set (SHARED_LIB_DIR ../../../shared-lib)
 
 include_directories (.
                      ../../runtime/include
                      ../../runtime/platform/include
                      ${SHARED_LIB_DIR}/include
-                     $ENV{SGX_SDK}/include)
+                     $ENV{SGX_SDK}/include
+                     $ENV{SGX_SDK}/include/tlibc
+                     $ENV{SGX_SDK}/include/libcxx)
 
 enable_language (ASM)
 

core/iwasm/products/linux-sgx/enclave-sample/Enclave/Enclave.edl

@@ -4,6 +4,8 @@
  */
 
 enclave {
+    from "sgx_tstdc.edl" import *;
+
     trusted {
         /* define ECALLs here. */
         public void ecall_iwasm_main(void);

core/iwasm/runtime/platform/linux-sgx/wasm_native.c

@@ -14,14 +14,8 @@
 #include "wasm_platform_log.h"
 #include "bh_common.h"
 
-#include <sys/ioctl.h>
-#include <sys/uio.h>
-#include <sys/syscall.h>
 #include <sys/types.h>
-#include <sys/stat.h>
 #include <unistd.h>
-#include <pwd.h>
-#include <fcntl.h>
 #include <errno.h>