This is a Linux-only feature that was already present as a
syscall. Add it just to linux/gnu for the moment, as the musl version
bundled in the Rust's x86_64-unknown-linux-musl toolchain doesn't
include it yet.

Signed-off-by: Sergio Lopez <slp@redhat.com>

fix #1633

memmem is a non-standard extension, first added in GNU libc and later
ported to other systems. Support for it is non-uniform, thus it was only
added to platforms that seem to support it.

This triggers a warning on a recent nightly, which in turn breaks CI due to
`#![deny(warnings)]` in libc-test/build.rs

This adds `utmpname(3)` on Linux with GNU libc.

Ref: https://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/baselib-utmpname-3.html

Exposes value for most unix like platforms

These constants have already been deprecated for a few releases with
a deprecation notice, so they can finally be removed.

Closes rust-lang/libc#665