Add port for RTEMS

[thesummer]

This is a port to the main branch of PR #3561.

I am currently porting the rust compiler and stdlib to the RTEMS realtime operating system (https://www.rtems.org/).
See #3561 for reference.
Once this is merged the attached changes also need to be added to libc to provide the target platform support.

The main resources for this port were:

• The RTEMS POSIX API Guide
• The RTEMS POSIX 1003.1 Compliance Guide
• The RTEMS Source Code Repository
• The newlib source Code Repository

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @JohnTitor (or someone else) some time within the next two weeks.

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[tgross35]

Thanks! I'll take a look at this once rust-lang/rust#127021 is available on a nightly.

[bors]

☔ The latest upstream changes (presumably #3872) made this pull request unmergeable. Please resolve the merge conflicts.

[tgross35]

That rust-lang/rust PR should merge soon, but until then

@rustbot blocked

[thesummer]

@tgross35 I rebased to current main.

@rustbot review

[tgross35]

I didn't take too close of a look at everything, but assuming tests pass this should be fine to get started. Thanks!

[thesummer]

Thanks, I took a look at existing ports and updated the missing values for types from newlib or RTEMS sources. I guess we will find some errors along the way once we test the port more thorougly with the nightly compiler.

[thesummer]

@tgross35 Sorry, I was a bit pre-occupied the last week and lost a bit track of the status here. Is there anything else I can help you with?

Edit: Commented the wrong PR. Meant to comment here: #3932 (comment) 🙈