internal/stdlib: update stdlib index for Go 1.26 Release Candidate 1

For golang/go#38706.

[git-generate]
GOTOOLCHAIN=go1.26rc1 go generate ./internal/stdlib

Change-Id: If211b4161a52a65eef416f25ed69306df89b2b8e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/730540
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

Author: mknyszek