internal/poll, internal/syscall/unix, net: enable writev on solaris

internal/poll, internal/syscall/unix, net: enable writev on solaris

The writev syscall is available since at least Solaris 11.3.

Reuse the existing illumos writev wrapper on solaris to implement
internal/poll.writev for net.(*netFD).writeBuffers.

Change-Id: I23adc3bb4637740c72bfb61bfa9697b432dfe3db
Reviewed-on: https://go-review.googlesource.com/c/go/+/427714
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>