unix: merge Linux readv/writev implementation with Darwin/OpenBSD

This reduces some code duplication.

Change-Id: Ib648f90a26960d0bc25d2fe27e9b3fb1419ac02d
Reviewed-on: https://go-review.googlesource.com/c/sys/+/777161
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>