arm64/arm64spec: skip build on 386 targets

Fix build failure:

..\..\gopath\src\\x\arch\arm64\arm64spec\spec.go:137: undefined: pdf.Outline
..\..\gopath\src\\x\arch\arm64\arm64spec\spec.go:148: undefined: pdf.Outline
..\..\gopath\src\\x\arch\arm64\arm64spec\spec.go:167: undefined: pdf.Page
..\..\gopath\src\\x\arch\arm64\arm64spec\spec.go:167: undefined: pdf.Reader
..\..\gopath\src\\x\arch\arm64\arm64spec\spec.go:425: undefined: pdf.Content
..\..\gopath\src\\x\arch\arm64\arm64spec\spec.go:425: undefined: pdf.Text
..\..\gopath\src\\x\arch\arm64\arm64spec\spec.go:616: undefined: pdf.Rect

arm64spec depends on which does not currently build on 386 platforms.

Updates golang/go#12840

Change-Id: I40de19c617f90bcf696586b57a4d09b1d307f0df
Reviewed-by: Brad Fitzpatrick <>
Run-TryBot: Brad Fitzpatrick <>
TryBot-Result: Gobot Gobot <>
diff --git a/arm64/arm64spec/spec.go b/arm64/arm64spec/spec.go
index ee784e5..1000d01 100644
--- a/arm64/arm64spec/spec.go
+++ b/arm64/arm64spec/spec.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
+// +build go1.6,amd64 go1.8
 // arm64spec reads the ``ARMv8-A Reference Manual''
 // to collect instruction encoding details and writes those
 // details to standard output in JSON format.