arm/armspec, vendor/rsc.io/pdf: require Go 1.6+ for use of vendored packages

Fixes Go 1.5 build failures on the dashboard like:
https://build.golang.org/log/97424fa33dc5535d387185597df669e625ea0e2c

Updates golang/go#11811

Change-Id: Iea03466d61a76d648d9e7ba16a9284d5f5438eb6
Reviewed-on: https://go-review.googlesource.com/24972
Reviewed-by: Andrew Gerrand <adg@golang.org>

arm/armspec/spec.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build go1.6
 // +build !386 go1.8
 // ... see golang.org/issue/12840
 

vendor/rsc.io/pdf/lex.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build go1.6
 // +build !386 go1.8
 // ... see golang.org/issue/12840
 

vendor/rsc.io/pdf/page.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build go1.6
 // +build !386 go1.8
 // ... see golang.org/issue/12840
 

vendor/rsc.io/pdf/ps.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build go1.6
 // +build !386 go1.8
 // ... see golang.org/issue/12840
 

vendor/rsc.io/pdf/read.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build go1.6
 // +build !386 go1.8
 // ... see golang.org/issue/12840