Kernel: Move PCSpeaker code to the x86-specific architecture directory

The PCSpeaker code is specific to x86 platforms, thus it makes sense to
put in the Arch/x86 subdirectory.

Kernel/Arch/x86/common/PCSpeaker.h

@@ -0,0 +1,13 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+class PCSpeaker {
+public:
+    static void tone_on(int frequency);
+    static void tone_off();
+};