Kernel: Add stubs for missing x86_64 functionality

This adds just enough stubs to make the kernel compile on x86_64. Obviously
it won't do anything useful - in fact it won't even attempt to boot because
Multiboot doesn't support ELF64 binaries - but it gets those compiler errors
out of the way so more progress can be made getting all the missing
functionality in place.

Kernel/Arch/x86/common/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 <AK/Types.h>
+#include <Kernel/Arch/x86/CPU.h>
+#include <Kernel/Arch/x86/Processor.h>
+#include <Kernel/Arch/x86/TrapFrame.h>
+#include <Kernel/KSyms.h>
+#include <Kernel/Process.h>
+
+void __assertion_failed(const char* msg, const char* file, unsigned line, const char* func)
+{
+    asm volatile("cli");
+    critical_dmesgln("ASSERTION FAILED: {}", msg);
+    critical_dmesgln("{}:{} in {}", file, line, func);
+
+    abort();
+}
+
+[[noreturn]] void abort()
+{
+    // Switch back to the current process's page tables if there are any.
+    // Otherwise stack walking will be a disaster.
+    auto process = Process::current();
+    if (process)
+        MM.enter_process_paging_scope(*process);
+
+    Kernel::dump_backtrace();
+    Processor::halt();
+
+    abort();
+}
+
+[[noreturn]] void _abort()
+{
+    asm volatile("ud2");
+    __builtin_unreachable();
+}