Kernel: Split SmapDisabler 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/SmapDisabler.h

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+
+namespace Kernel {
+
+class SmapDisabler {
+public:
+    SmapDisabler();
+    ~SmapDisabler();
+
+private:
+    const FlatPtr m_flags;
+};
+
+}