Rust std on ThreadX - an RP2040 demo

It won't be on my RustConf slides, but I now have a Rust standard library demo working on a Raspberry Pi Pico board, with a RP2040 MCU. It's in the demos repo, along with the other QEMU and i.MX RT demos.

As with all the demos so far, it's a prototype, just blinking an LED and using USB serial for stdout. I built on the rp-pico board support package to simplify hardware setup. Even though I'm not familiar with these MCUs, bringup was straightforward!

You'll need a new toolchain with the thumbv6m-threadx-eabi target. I updated the distribution page to describe how to get set up with all supported ThreadX targets. If you're already set up with the toolchain, pull the new commits from my fork (on the threadx branch), then include the new target in your build, like what's shown below.

./x build --target=thumbv7em-threadx-eabihf,thumbv6m-threadx-eabi,x86_64-unknown-linux-gnu