Kernel: Simplify reboot & poweroff code flow a bit

Instead of using ifdefs to use the correct platform-specific methods, we
can just use the same pattern we use for the microseconds_delay function
which has specific implementations for each Arch CPU subdirectory.

When linking a kernel image, the actual correct and platform-specific
power-state changing methods will be called in Firmware/PowerState.cpp
file.

Kernel/Arch/PowerState.h

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2023, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+
+namespace Kernel {
+
+void arch_specific_reboot();
+void arch_specific_poweroff();
+
+}