Kernel: Share Processor class (and others) across architectures

About half of the Processor code is common across architectures, so
let's share it with a templated base class. Also, other code that can be
shared in some ways, like FPUState and TrapFrame functions, is adjusted
here. Functions which cannot be shared trivially (without internal
refactoring) are left alone for now.

Kernel/Arch/x86_64/FPUState.h

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2023, kleines Filmröllchen <filmroellchen@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Platform.h>
+#include <AK/Types.h>
+#include <Kernel/Arch/x86_64/SIMDState.h>
+
+VALIDATE_IS_X86()
+
+namespace Kernel {
+
+struct [[gnu::aligned(64), gnu::packed]] FPUState {
+    SIMD::LegacyRegion legacy_region;
+    SIMD::Header xsave_header;
+
+    // FIXME: This should be dynamically allocated! For now, we only save the `YMM` registers here,
+    // so this will do for now. The size of the area is queried via CPUID(EAX=0dh, ECX=2):EAX.
+    // https://www.intel.com/content/dam/develop/external/us/en/documents/36945
+    u8 ext_save_area[256];
+};
+
+}