Kernel: Move File.{cpp,h} into FileSystem/

Also tweak the kernel's Makefile to use -nostdinc and -nostdinc++.
This prevents us from picking up random headers from ../Root, which may
include older versions of kernel headers.

Since we still need <initializer_list> for Vector, we specifically include
the necessary GCC path. This is a bit hackish but it works for now.

Kernel/FileSystem/InodeFile.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include <Kernel/File.h>
+#include <Kernel/FileSystem/File.h>
 
 class Inode;