[illumos] use pipe2 to create anonymous pipes

pipe2 allows the newly-created pipe to atomically be CLOEXEC.

pipe2 was added to illumos a long time ago:
illumos/illumos-gate@5dbfd19.
I've verified that this change passes all tests.

library/std/src/sys/pal/unix/pipe.rs

@@ -23,6 +23,7 @@ pub fn anon_pipe() -> io::Result<(AnonPipe, AnonPipe)> {
             target_os = "dragonfly",
             target_os = "freebsd",
             target_os = "hurd",
+            target_os = "illumos",
             target_os = "linux",
             target_os = "netbsd",
             target_os = "openbsd",