os, internal/syscall/unix: consolidate Pipe implementations

All platforms with the pipe2 syscall now provide syscall.Pipe2.  Use it
to implement os.Pipe.

This also allows to drop the illumos-specific wrapper in
internal/sys/unix.

Change-Id: Ieb712a1498e86a389bad261e4e97c61c11d4bdd0
Reviewed-on: https://go-review.googlesource.com/c/go/+/390715


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>