std: Fix usage of SOCK_CLOEXEC

This code path was intended to only get executed on Linux, but unfortunately the
`cfg!` was malformed so it actually never got executed.

Author: alexcrichton

src/libstd/sys/unix/net.rs

@@ -67,7 +67,7 @@ impl Socket {
             // this option, however, was added in 2.6.27, and we still support
             // 2.6.18 as a kernel, so if the returned error is EINVAL we
             // fallthrough to the fallback.
-            if cfg!(linux) {
+            if cfg!(target_os = "linux") {
                 match cvt(libc::socket(fam, ty | SOCK_CLOEXEC, 0)) {
                     Ok(fd) => return Ok(Socket(FileDesc::new(fd))),
                     Err(ref e) if e.raw_os_error() == Some(libc::EINVAL) => {}
@@ -87,7 +87,7 @@ impl Socket {
             let mut fds = [0, 0];
 
             // Like above, see if we can set cloexec atomically
-            if cfg!(linux) {
+            if cfg!(target_os = "linux") {
                 match cvt(libc::socketpair(fam, ty | SOCK_CLOEXEC, 0, fds.as_mut_ptr())) {
                     Ok(_) => {
                         return Ok((Socket(FileDesc::new(fds[0])), Socket(FileDesc::new(fds[1]))));