Added stubs for stdio.h functions

putchar() and puts() are platform-dependent

src/stdio.c

@@ -0,0 +1,11 @@
+#include "stdio.h"
+
+int printf(const char *format, ...)
+{
+
+}
+
+int sprintf(char *str, const char *format, ...)
+{
+    
+}

src/x86/putc.c

@@ -0,0 +1,11 @@
+#include "stdio.h"
+
+int putchar(int c)
+{
+
+}
+
+int puts(const char *str)
+{
+    
+}