crypto/tls: add linkname comments dropped from CL 587220

crypto/tls: add linkname comments dropped from CL 587220

A bad merge syncing before the submit of CL 587220 dropped these.
(I forgot to write the file out.)

For #67401.

Change-Id: I6f2ba69f388907f3d24eeef55c80cbb2cf51f580
Reviewed-on: https://go-review.googlesource.com/c/go/+/587755
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Marten Seemann <martenseemann@gmail.com>