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/aarch64/SmapDisabler.cpp

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/Arch/SmapDisabler.h>
+
+namespace Kernel {
+
+SmapDisabler::SmapDisabler()
+    : m_flags(0)
+{
+}
+
+SmapDisabler::~SmapDisabler()
+{
+}
+
+}