unix: add missing HWTSTAMP_* constants

Change-Id: I72e3c04afda8e9382fedc6aac6cf2824da82e1dc
GitHub-Last-Rev: ae9f762bcd6b332f65b012c57abcd238018a242a
GitHub-Pull-Request: golang/sys#283
Reviewed-on: https://go-review.googlesource.com/c/sys/+/805700
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: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>