Add missing arch_zephyr.h, add musl target

src/arch_zephyr.h

@@ -0,0 +1,27 @@
+#pragma once
+
+#if MG_ARCH == MG_ARCH_ZEPHYR
+
+#include <zephyr.h>
+
+#include <ctype.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <net/socket.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>
+
+#define strerror(x) zsock_gai_strerror(x)
+#define FD_CLOEXEC 0
+#define F_SETFD 0
+
+int rand(void);
+int sscanf(const char *, const char *, ...);
+
+#endif