Add initial Rpi4 support

[U007D]

Description

• I built a patched version of QEMU with RPi4 support (see https://github.com/U007D/qemu-raspi4)
• Updated the Docker image to use this build (see https://hub.docker.com/repository/docker/u007d/osdev-utils-rpi4/general)
• Updated references in this repo to use updated Docker image (feel free to clone/update your Docker image and switch the references back to your image).

Pre-commit steps

• Tested on QEMU and real HW Rasperry Pi.
  • rpi4 did not pass 03_hacky_hello_world on QEMU. This may or may not be expected given the state of the QEMU patch, or given the differences in rpi3 and rpi4, but I thought I would share the work in case this was an issue you were familiar with.
• Ran ./contributor_setup.sh followed by ./devtool ready_for_publish
  • You'll need Ruby with Bundler and NPM installed locally.
  • If no Rust-related files were changed, ./devtool ready_for_publish_no_rust can be used instead (faster).
  • This step is optional, but much appreciated if done.