Kernel: Reorganize Arch/x86 directory to Arch/x86_64 after i686 removal

No functional change.

Kernel/Arch/x86_64/CPU.cpp

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Assertions.h>
+#include <Kernel/Arch/CPU.h>
+#include <Kernel/Panic.h>
+#include <Kernel/Process.h>
+
+using namespace Kernel;
+
+void __assertion_failed(char const* msg, char const* file, unsigned line, char const* func)
+{
+    asm volatile("cli");
+    critical_dmesgln("ASSERTION FAILED: {}", msg);
+    critical_dmesgln("{}:{} in {}", file, line, func);
+
+    abort();
+}
+
+[[noreturn]] void abort()
+{
+    // Avoid lock ranking checks on crashing paths, just try to get some debugging messages out.
+    auto thread = Thread::current();
+    if (thread)
+        thread->set_crashing();
+
+    // Switch back to the current process's page tables if there are any.
+    // Otherwise stack walking will be a disaster.
+    if (Process::has_current())
+        Memory::MemoryManager::enter_process_address_space(Process::current());
+
+    PANIC("Aborted");
+}
+
+[[noreturn]] void _abort()
+{
+    asm volatile("ud2");
+    __builtin_unreachable();
+}