feat(io): implement printf and strlen

include/io.h

@@ -0,0 +1,14 @@
+#ifndef RIVOS_IO_H
+#define RIVOS_IO_H
+
+#include "../include/types.h"
+
+#define va_list __builtin_va_list
+#define va_start __builtin_va_start
+#define va_end __builtin_va_end
+#define va_arg __builtin_va_arg
+
+void r_printf(const char *fmt, ...);
+size_t r_strlen(const char *str);
+
+#endif

include/kernel.h

@@ -0,0 +1,9 @@
+#ifndef RIVOS_KERNEL_H
+#define RIVOS_KERNEL_H
+
+typedef struct sbiret_s {
+	long error;
+	long value;
+} sbiret_s;
+
+#endif