cmd/link: don't update offset of existing ELF section name

cmd/link: don't update offset of existing ELF section name

Fixes #76656

Change-Id: If2e823ba1577700af00f5883e4ea5c139e4749c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/726100
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>