runtime, syscall: implement syscall.Pipe using syscall.Pipe2 on solaris

All other platforms providing the pipe2 syscall already implement it
that way. Do so as well on solaris, which allows to drop
runtime.syscall_pipe and its dependencies as well.

Change-Id: Icf04777f21d1804da74325d173fefdc87caa42eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/390716


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>