cmd/coordinator, dashboard: trim benchmark builders for initial launch

- Enable benchmarks on only linux-amd64, with 3 additional test
  helpers, to ensure we fit in the 5m trybot budget
  (disabling freebsd-amd64-gce101, linux-386, linux-arm,
  windows-amd64-2008, and windows-386-2008).
- Run benchmarks with GOMAXPROCS=2 to reduce variation in
  benchmarks. See
  https://perf.golang.org/search?q=upload%3A20170512.4+cores%3Aall+parallel%3A4+%7C+gomaxprocs%3A2+vs+gomaxprocs%3A16+vs+gomaxprocs%3A32

Updates golang/go#19871

Change-Id: I8a28127be86c0c8691605cae4c9d603e3c8122cc
Reviewed-on: https://go-review.googlesource.com/43532
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Avelino <t@avelino.xxx>
2 files changed