arm/armspec, vendor/rsc.io/pdf: don't build on 386 until Go 1.8

This won't be fixed until the SSA backend in Go 1.8, so don't even try
to build. It just clutters the dashboard with red.

Updates golang/go#12840 (out of fixed registers)
Updates golang/go#11811 (subrepos need to be green)

Change-Id: I25b3db4d3bd47d16e7e42a55429b343dc27f9f43
Reviewed-on: https://go-review.googlesource.com/24942
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
5 files changed