To speed up the ssacheck check builder and make it on by default as a
trybot.

Change-Id: I91a3347491507c84f4878dff744ca426ba3e2e9f
Reviewed-on: https://go-review.googlesource.com/22755
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>