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/ScopedCritical.h

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Noncopyable.h>
+
+namespace Kernel {
+
+class ScopedCritical {
+    AK_MAKE_NONCOPYABLE(ScopedCritical);
+
+public:
+    ScopedCritical();
+    ~ScopedCritical();
+    ScopedCritical(ScopedCritical&& from);
+
+    ScopedCritical& operator=(ScopedCritical&& from);
+
+    void leave();
+    void enter();
+
+private:
+    bool m_valid { false };
+};
+
+}