Fix the output of build -n when adding to an existing archive with the
gc toolchain by observing that we are, now, always doing that.  When
using the gc toolchain the archive is now always created by the Go
compiler, and never by the pack command.

No test because we have not historically tested build -n output.

Fixes #13118.

Change-Id: I3a5c43cf45169fa6c9581e4741309c77d2b6e58b
Reviewed-on: https://go-review.googlesource.com/16761Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>