cmd/go: for missing binary-only package, say where it should be

Before this CL the user effectively has to guess at the expected
location of a binary-only package. While the location is normally
obvious ($GOPATH/pkg/GOOS_GOARCH/PATH/PKG.a) it is much less so when
building with options that implicitly add an -installsufix option.

Fixes #26590

Change-Id: I753ef54d6dcf733bb456dba65a4a92e4db57a1b0
Reviewed-on: https://go-review.googlesource.com/125818


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>