Updates #11918

Replace calls to lchown(2) with fchownat(2) for linux/arm64 as the former
is not suppored.

This is a companion to CL 12833 which is ready to be applied to the main
repo, and verified by CL 12834 which can be applied once this change and
CL 12833 have landed.

Change-Id: I63de3d0da1e4b4e1f253e51e0ed8e0cd0e56d63a
Reviewed-on: https://go-review.googlesource.com/12837Reviewed-by: Rob Pike <r@golang.org>