Userland faulted on the very first instruction before because the
PML4T/PDPT/etc. weren't marked as user-accessible. For some reason
x86 doesn't care about that.
Also, we need to provide an appropriate userspace stack segment
selector to iretq.
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.
