Kernel: Factor our PreviousMode into RegisterState::previous_mode

Various places in the kernel were manually checking the cs register for
x86_64, however to share this with aarch64 a function in RegisterState
is added, and the call-sites are updated. While we're here the
PreviousMode enum is renamed to ExecutionMode.

Kernel/ExecutionMode.h

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+
+namespace Kernel {
+
+enum class ExecutionMode : u8 {
+    Kernel = 0,
+    User,
+};
+
+}