add RT-Thread support

src/arch_rtthread.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#if MG_ARCH == MG_ARCH_RTTHREAD
+
+#include <rtthread.h>
+#include <ctype.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <sys/socket.h>
+#include <sys/select.h>
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
+#include <time.h>
+
+#ifndef MG_IO_SIZE
+#define MG_IO_SIZE 1460
+#endif
+
+#endif // MG_ARCH == MG_ARCH_RTTHREAD