Add port for RTEMS

[thesummer]

I am currently porting the rust compiler and stdlib to the RTEMS realtime operating system (https://www.rtems.org/).

This PR is the prerequisite for my changes to the rust repo.
style.sh succeeds, the libc-test fails to compile on my machine, but since I only add things which are behind guards for target_os = "rtems". I hope CI will give a 👍 .

Once this passed. I will also add a PR for main branch.

[rustbot]

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[JohnTitor]

Thanks! Could you add the target to rustc first? This crate follows rustc's platform support for adding/removing targets.

[thesummer]

Ah, ok. Will do.