Stubs, Serial Console, UEFI, and Build Tools for aarch64

[PandaZ3D]

This PR begins a series of patches that implement a full port of Twizzler to ARM (targeting aarch64/ARMv8). A number of changes had to be implemented to support the aarch64 target and make the kernel more architecture neutral.

• make sure that x86 only crates are not compiled
• add conditional compilation for code in arch
• change build process in xtask to compile for aarch64
• add stubs for architechture dependent code in twizzler-abi and arch
• boot the kernel using Limine/UEFI for aarch64
• implement a driver for a PL011 UART

The result is a simple kernel that can boot with UEFI via Limine and print to the serial console using UART.

[dbittman]

For the VirtAddr and PhysAddr types, I think it would be better to require the arch-dep code to implement them, and then re-export them from inside crate::memory.

[PandaZ3D]

require the arch-dep code to implement them, and then re-export them from inside crate::memory.

I'm on board with that. Where should we document this interface?