cmd/link: require cgo for -linkmode=external test

cmd/link: require cgo for -linkmode=external test

For #71416
Fixes #71957

Change-Id: I2180dada34d9dd2d3f5b0aaf8525951fd2e86a27
Reviewed-on: https://go-review.googlesource.com/c/go/+/652277
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>