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/Firmware/PowerState.cpp

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2023, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Format.h>
+#include <Kernel/Arch/PowerState.h>
+#include <Kernel/FileSystem/FileSystem.h>
+#include <Kernel/Firmware/ACPI/Parser.h>
+#include <Kernel/Firmware/PowerState.h>
+#include <Kernel/TTY/ConsoleManagement.h>
+#include <Kernel/Tasks/Process.h>
+
+namespace Kernel::Firmware {
+
+void reboot()
+{
+    MutexLocker locker(Process::current().big_lock());
+
+    dbgln("acquiring FS locks...");
+    FileSystem::lock_all();
+    dbgln("syncing mounted filesystems...");
+    FileSystem::sync();
+    dbgln("attempting reboot via ACPI");
+    if (ACPI::is_enabled())
+        ACPI::Parser::the()->try_acpi_reboot();
+
+    arch_specific_reboot();
+
+    dbgln("reboot attempts failed, applications will stop responding.");
+    dmesgln("Reboot can't be completed. It's safe to turn off the computer!");
+    Processor::halt();
+}
+
+void poweroff()
+{
+    MutexLocker locker(Process::current().big_lock());
+
+    ConsoleManagement::the().switch_to_debug();
+
+    dbgln("acquiring FS locks...");
+    FileSystem::lock_all();
+    dbgln("syncing mounted filesystems...");
+    FileSystem::sync();
+
+    dbgln("attempting system shutdown...");
+    arch_specific_poweroff();
+
+    dbgln("shutdown attempts failed, applications will stop responding.");
+    dmesgln("Shutdown can't be completed. It's safe to turn off the computer!");
+    Processor::halt();
+}
+
+}

Kernel/Firmware/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::Firmware {
+
+void reboot();
+void poweroff();
+
+}