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/x86_64/InterruptEntry.cpp

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021, Gunnar Beutner <gbeutner@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/Arch/x86/DescriptorTable.h>
+#include <Kernel/Arch/x86/TrapFrame.h>
+
+// clang-format off
+asm(
+".globl interrupt_common_asm_entry\n"
+"interrupt_common_asm_entry: \n"
+"    int3 \n" // FIXME
+".globl common_trap_exit \n"
+"common_trap_exit: \n"
+// another thread may have handled this trap at this point, so don't
+// make assumptions about the stack other than there's a TrapFrame
+// and a pointer to it.
+"    call exit_trap \n"
+"    int3 \n" // FIXME
+);
+// clang-format on