Kernel: Move deferred call code into separate DeferredCallPool class

This allows us to share this code between the x86_64 and aarch64 build.

Kernel/Arch/DeferredCallPool.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2020, Tom <tomut@yahoo.com>
+ * Copyright (c) 2023, Timon Kruiper <timonkruiper@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <Kernel/Arch/DeferredCallEntry.h>
+
+namespace Kernel {
+
+class DeferredCallPool {
+public:
+    void init();
+    void execute_pending();
+    DeferredCallEntry* get_free();
+    void return_to_pool(DeferredCallEntry*);
+    void queue_entry(DeferredCallEntry*);
+
+private:
+    DeferredCallEntry* m_pending_deferred_calls; // in reverse order
+    DeferredCallEntry* m_free_deferred_call_pool_entry;
+    DeferredCallEntry m_deferred_call_pool[5];
+};
+
+}