Support for SMP on x86_64.

No major changes to the kernel: just compiled in arch_smp.cpp and fixed the
IDT load in arch_cpu_init_percpu to use the correct limit for x86_64 (uses
sizeof(interrupt_descriptor)). In the boot loader, changed smp_boot_other_cpus
to construct a temporary GDT and get the page directory address from CR3, as
what's in kernel_args will be 64-bit stuff and will not work to switch the
CPUs into 32-bit mode in the trampoline code. Refactored 64-bit kernel entry
code to not use the stack after disabling paging, as the secondary CPUs are
given a 32-bit virtual stack address by the SMP trampoline code which will
no longer work.

Some fixes to the long mode switch code.

* Wasn't storing the fixed virtual address of the PML4 in kernel_args.
* After switching to long mode, reload GDTR with the virtual address of
the GDT. This was working fine until now because the physical address
was identity mapped, but broke as soon as I removed the identity
mapping.

Implemented long mode setup/switch code, the bootloader can now start the 64-bit kernel!

The setup procedure is fairly simple: create a 64-bit GDT and 64-bit page
tables that include all kernel mappings from the 32-bit address space, but at
the correct 64-bit address, then go through kernel_args and changes all virtual
addresses to 64-bit addresses, and finally switch to long mode and jump to the
kernel.

Support for SMP on x86_64.

No major changes to the kernel: just compiled in arch_smp.cpp and fixed the
IDT load in arch_cpu_init_percpu to use the correct limit for x86_64 (uses
sizeof(interrupt_descriptor)). In the boot loader, changed smp_boot_other_cpus
to construct a temporary GDT and get the page directory address from CR3, as
what's in kernel_args will be 64-bit stuff and will not work to switch the
CPUs into 32-bit mode in the trampoline code. Refactored 64-bit kernel entry
code to not use the stack after disabling paging, as the secondary CPUs are
given a 32-bit virtual stack address by the SMP trampoline code which will
no longer work.

Some fixes to the long mode switch code.

* Wasn't storing the fixed virtual address of the PML4 in kernel_args.
* After switching to long mode, reload GDTR with the virtual address of
the GDT. This was working fine until now because the physical address
was identity mapped, but broke as soon as I removed the identity
mapping.

Implemented long mode setup/switch code, the bootloader can now start the 64-bit kernel!

The setup procedure is fairly simple: create a 64-bit GDT and 64-bit page
tables that include all kernel mappings from the 32-bit address space, but at
the correct 64-bit address, then go through kernel_args and changes all virtual
addresses to 64-bit addresses, and finally switch to long mode and jump to the
kernel.