Kernel: Split ScopedCritical so header is platform independent

A new header file has been created in the Arch/ folder while the
implementation has been moved into a CPP living in the X86 folder.

Kernel/Arch/aarch64/ScopedCritical.cpp

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/Arch/ScopedCritical.h>
+
+#include <Kernel/Arch/Processor.h>
+
+namespace Kernel {
+
+ScopedCritical::ScopedCritical()
+{
+}
+
+ScopedCritical::~ScopedCritical()
+{
+}
+
+ScopedCritical::ScopedCritical(ScopedCritical&& /*from*/)
+{
+}
+
+ScopedCritical& ScopedCritical::operator=(ScopedCritical&& /*from*/)
+{
+    return *this;
+}
+
+void ScopedCritical::leave() { }
+
+void ScopedCritical::enter() { }
+
+}