os: define wrapSyscallError only on linux

os: define wrapSyscallError only on linux

wrapSyscallError by now is only used on linux in the methods defined in
os/zero_copy_linux.go. Move the definition there.

Change-Id: I0ca5749adaac44e8d095b8452458b647f595d3c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/575595
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>