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/Spinlock.h

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Platform.h>
+
+#if ARCH(X86_64) || ARCH(I386)
+#    include <Kernel/Arch/x86/Spinlock.h>
+#elif ARCH(AARCH64)
+#    include <Kernel/Arch/aarch64/Spinlock.h>
+#else
+#    error "Unknown architecture"
+#endif