cmd/link: avoid stamping runtime.defaultGOROOT when paths are being trimmed

cmd/link: avoid stamping runtime.defaultGOROOT when paths are being trimmed

Previously, runtime.GOROOT() would return the string "go" in a binary
build with -trimpath. This change stamps the empty string instead,
using a sentinel value passed from cmd/go that looks like the GOROOT
environment variable (either "$GOROOT" or "%GOROOT%", depending on the
platform).

Fixes #51461

Change-Id: I1f10ef2435016a7b6213bd8c547df911f7feeae7
Reviewed-on: https://go-review.googlesource.com/c/go/+/390024
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>