Parallel to CL 151938 for package x/sys/unix instead of syscall.

iOS needs to use these functions without the "64" postfix.
(The functions do exist, but the App Store bans their use.)

Updates golang/go#28984

Change-Id: I6b82950700cc8a1afca612844b05fa007574e008
Reviewed-on: https://go-review.googlesource.com/c/154658Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>