os, internal/syscall/unix: use pipe2 instead of pipe on illumos

Illumos provides the pipe2 syscall. Add a wrapper to
internal/syscall/unix and use it to implement os.Pipe.

Change-Id: I26ecdbcae1e8d51f80e2bc8a86fb129826387b1f
Reviewed-on: https://go-review.googlesource.com/c/go/+/254981


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>