LibC: Avoid ninja-imports of system functions

This adds a new header <sys/internals.h>, which provides access to LibC internals.
This is in the interest of type-checking LibC itself, as well as enabling less-hacky
access for uses like LinkDemo.

And, of course, this progresses LibC towards building cleanly with -Wmissing-declarations.

Libraries/LibC/stdio.cpp

@@ -38,6 +38,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/internals.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <unistd.h>