Fix failing build of std on armv5te-unknown-linux-uclibceabi due to missing cmsg_len_zero

[operutka]

I'm getting the following error when trying to build std on armv5te-unknown-linux-uclibceabi:

error[E0425]: cannot find value `cmsg_len_zero` in this scope
   --> /home/operutka/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/unix/ext/net/ancillary.rs:376:47
    |
376 |             let data_len = (*cmsg).cmsg_len - cmsg_len_zero;
    |                                               ^^^^^^^^^^^^^ not found in this scope

Obviously, this branch:

cfg_if::cfg_if! {
    if #[cfg(any(target_os = "android", all(target_os = "linux", target_env = "gnu")))] {
        let cmsg_len_zero = libc::CMSG_LEN(0) as libc::size_t;
    } else if #[cfg(any(
                  target_os = "dragonfly",
                  target_os = "emscripten",
                  target_os = "freebsd",
                  all(target_os = "linux", target_env = "musl",),
                  target_os = "netbsd",
                  target_os = "openbsd",
              ))] {
        let cmsg_len_zero = libc::CMSG_LEN(0) as libc::socklen_t;
    }
}

does not cover the case all(target_os = "linux", target_env = "uclibc").

[rust-highfive]

r? @dtolnay

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[dtolnay]

Thanks!

[dtolnay]

@bors r+

[bors]

📌 Commit ec07815 has been approved by dtolnay

[bors]

⌛ Testing commit ec07815 with merge c813545...

[bors]

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing c813545 to master...