Kernel/riscv64: Make the kernel compile

This commits inserts TODOs into all necessary places to make the kernel
compile on riscv64!

Kernel/Arch/riscv64/PageDirectory.cpp

@@ -0,0 +1,65 @@
+/*
+ * Copyright (c) 2023, Sönke Holz <sholz8530@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Format.h>
+#include <AK/Singleton.h>
+
+#include <Kernel/Arch/PageDirectory.h>
+#include <Kernel/Library/LockRefPtr.h>
+#include <Kernel/Sections.h>
+
+namespace Kernel::Memory {
+
+void PageDirectory::register_page_directory(PageDirectory*)
+{
+    TODO_RISCV64();
+}
+
+void PageDirectory::deregister_page_directory(PageDirectory*)
+{
+    TODO_RISCV64();
+}
+
+ErrorOr<NonnullLockRefPtr<PageDirectory>> PageDirectory::try_create_for_userspace(Process&)
+{
+    TODO_RISCV64();
+}
+
+LockRefPtr<PageDirectory> PageDirectory::find_current()
+{
+    TODO_RISCV64();
+}
+
+void activate_kernel_page_directory(PageDirectory const&)
+{
+    TODO_RISCV64();
+}
+
+void activate_page_directory(PageDirectory const&, Thread*)
+{
+    TODO_RISCV64();
+}
+
+UNMAP_AFTER_INIT NonnullLockRefPtr<PageDirectory> PageDirectory::must_create_kernel_page_directory()
+{
+    return adopt_lock_ref_if_nonnull(new (nothrow) PageDirectory).release_nonnull();
+}
+
+PageDirectory::PageDirectory() = default;
+
+UNMAP_AFTER_INIT void PageDirectory::allocate_kernel_directory()
+{
+    TODO_RISCV64();
+}
+
+PageDirectory::~PageDirectory()
+{
+    if (is_root_table_initialized()) {
+        deregister_page_directory(this);
+    }
+}
+
+}