[release-branch.go1.25] cmd/go: remove user-content from doc strings in cgo ASTs.

[release-branch.go1.25] cmd/go: remove user-content from doc strings in cgo ASTs.

Thank you to RyotaK (https://ryotak.net) of GMO Flatt Security Inc. for reporting this issue.

Updates #76697
Fixes #77129
Fixes CVE-2025-61732

Change-Id: I9ecbef556f6e545fb152407041cd086c069f22d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/740040
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>