Add support Zephyr OS

[tomtom5152]

Zephyr is an embedded RTOS that provides its own POSIX API utilising newlib. Adding support to libc is the first step in providing full rust support for the OS.

[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) soon.

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[bors]

☔ The latest upstream changes (presumably #3209) made this pull request unmergeable. Please resolve the merge conflicts.

[tomtom5152]

Rebased onto master to resolve conflicts and noticed @JohnTitor has added the S-blocked tag to this. Is there any guidance on what is needed to resolve the blocked status whilst I working on greater compiler integration please?

[JohnTitor]

Is there any guidance on what is needed to resolve the blocked status whilst I working on greater compiler integration please?

We haven't established a policy about adding a new target, and we only accept targets if an MCP is approved (there are some reasons that we do not accept targets unconditionally, e.g. we have to review if a target is legally valid, it's difficult to determine if we can drop the support, etc...). See #3032 for the previous discussion.

[JohnTitor]

The MCP has been approved, this now needs to resolve merge conflicts.

[JohnTitor]

Triage: Closing as inactive, feel free to re-submit a new PR if you're still interested in this change. Thank you anyway!