[dev.link] cmd/link: relocating Errorf() to ErrorReporter
Add an Errorf method to ErrorReporter. The hope is that we can
consolidate error handling/reporting in this helper, and eventually
do away with Link.Errorf and the global Errorf function (which
can be removed once we've eliminated enough uses of *sym.Symbol).
Change-Id: Ie1147020b8409b9c57acfd71c942b287b214afca
Reviewed-on: https://go-review.googlesource.com/c/go/+/224380 Reviewed-by: Jeremy Faller <jeremy@golang.org>