os, syscall: use unix build tag where appropriate

os, syscall: use unix build tag where appropriate

These newly added files may use the unix build tag instead of explitly
listing all unix-like GOOS values.

For #51572

Change-Id: I31c71d2b5533b39bbccd89bf616a99b8e33565d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/651996
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>