cmd/link: require cgo support for TestSectionsWithSameName

The test doesn't really require cgo, but it does require that we know
the right flags to use to run the C compiler, and that is not
necessarily correct if we don't support cgo.

Fixes #31565

Change-Id: I04dc8db26697caa470e91ad712376aa621cf765d
Reviewed-on: https://go-review.googlesource.com/c/go/+/172981


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>