Add ThreadX support

[raphaelmeyer]

Add basic support for ThreadX:

• threads
• semaphores
• mutexes

[haneefdm]

Since this is a new OS, could you emulate what other OSes have had to do

1. Update README.md with yourself as the contributor
2. Update CHANGELOG.md
3. Create a test case like I suggested to all the other OS contributors

See:
#76
#75

Sorry, these are not documented already.

[raphaelmeyer]

Create a test case like I suggested to all the other OS contributors
My test setup is at https://github.com/raphaelmeyer/pico-threadx

[haneefdm]

Just emerged. I am waiting on one more Pending PR, and it is a big one.

#73

That is for FreeRTOS. We will have to make a similar change to all RTOSes. Especially the stack pointer for RUNNING task. MPU support will be dependent on when each RTOS adds such support and when contributors have the time to update this extension.

When that is done, I will make a release. I can make a private build if someone requests. You don't need it because you already know how to build