add 'environ' variable

[KaiTomotake]

Description

Add the environ variable to unix/mod.rs.

Sources

https://github.com/bminor/glibc/blob/872cc459a9ec2a082cf1c292246708c7327d55e9/posix/environ.c#L9

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

Fixes #2520

[KaiTomotake]

FreeBSD CI failed due to crates.io SSL certificate verification error.
This appears to be an infrastructure issue unrelated to this PR.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[JohnTitor]

Does macOS have it in the headers? Any source?

[KaiTomotake]

I investigated this further, but I wasn’t able to confirm sufficiently reliable primary sources regarding environ on my own. Given this uncertainty, I believe it would be risky to add it to libc as-is, so I plan to close this PR for now. If clearer evidence or assumptions can be established in the future, I would be happy to revisit it.
I am truly sorry.