LibC: add probably functional *printf

I wrote a general printf function that takes an putc function
pointer. We can use this to implement all the printf family
functions. I haven't done thorough testing with this, but it seems
to be functional for the most part

libc/include/printf_impl.h

@@ -0,0 +1,11 @@
+#pragma once
+
+#include <sys/cdefs.h>
+#include <stdarg.h>
+#include <stddef.h>
+
+__BEGIN_DECLS
+
+int printf_impl(const char* format, va_list arguments, int (*putc_fun)(int, void*), void* data);
+
+__END_DECLS