app/build: add perf subrepo For golang/go#14304. Change-Id: I4b9737716bdb9de74fd9ca5e855f853494651b77 Reviewed-on: https://go-review.googlesource.com/34614 Run-TryBot: Russ Cox <rsc@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>