Kernel: Move spinlock into Arch

Spinlocks are tied to the platform they are built for, this is why they
have been moved into the Arch folder. They are still available via
"Locking/Spinlock.h"

An Aarch64 stub has been created

Kernel/Arch/aarch64/Spinlock.h

@@ -0,0 +1,78 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Noncopyable.h>
+#include <AK/Types.h>
+#include <Kernel/Locking/LockRank.h>
+
+namespace Kernel {
+
+class Spinlock {
+    AK_MAKE_NONCOPYABLE(Spinlock);
+    AK_MAKE_NONMOVABLE(Spinlock);
+
+public:
+    Spinlock(LockRank rank = LockRank::None)
+    {
+        (void)rank;
+    }
+
+    ALWAYS_INLINE u32 lock()
+    {
+        return 0;
+    }
+
+    ALWAYS_INLINE void unlock(u32 /*prev_flags*/)
+    {
+    }
+
+    [[nodiscard]] ALWAYS_INLINE bool is_locked() const
+    {
+        return false;
+    }
+
+    ALWAYS_INLINE void initialize()
+    {
+    }
+};
+
+class RecursiveSpinlock {
+    AK_MAKE_NONCOPYABLE(RecursiveSpinlock);
+    AK_MAKE_NONMOVABLE(RecursiveSpinlock);
+
+public:
+    RecursiveSpinlock(LockRank rank = LockRank::None)
+    {
+        (void)rank;
+    }
+
+    ALWAYS_INLINE u32 lock()
+    {
+        return 0;
+    }
+
+    ALWAYS_INLINE void unlock(u32 /*prev_flags*/)
+    {
+    }
+
+    [[nodiscard]] ALWAYS_INLINE bool is_locked() const
+    {
+        return false;
+    }
+
+    [[nodiscard]] ALWAYS_INLINE bool is_locked_by_current_processor() const
+    {
+        return false;
+    }
+
+    ALWAYS_INLINE void initialize()
+    {
+    }
+};
+
+}