Kernel: Move ThreadRegisters into arch-specific directory

These are architecture-specific anyway, so they belong in the Arch
directory. This commit also adds ThreadRegisters::set_initial_state to
factor out the logic in Thread.cpp.

Kernel/Arch/aarch64/ThreadRegisters.h

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2022, Timon Kruiper <timonkruiper@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+#include <Kernel/Memory/AddressSpace.h>
+
+namespace Kernel {
+
+struct ThreadRegisters {
+    u64 x[31];
+    u64 elr_el1;
+    u64 sp_el0;
+
+    FlatPtr ip() const { return elr_el1; }
+    void set_ip(FlatPtr value) { elr_el1 = value; }
+
+    void set_sp(FlatPtr value) { sp_el0 = value; }
+
+    void set_initial_state(bool, Memory::AddressSpace&, FlatPtr kernel_stack_top)
+    {
+        set_sp(kernel_stack_top);
+    }
+};
+
+}