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/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>