Change aarch64 example to use aarch64-cpu and aarch64-rt

[jonathanpallant]

There are CPU and runtime crates for aarch64, and it would be a better example to show people how to use those, rather than doing the bare minimum boot-strap that doesn't even set up an interrupt vector table correctly. It should also use the semihosting crate for text output and program termination.

[jonathanpallant]

It could also use the arm-gic driver to show interrupt handling.