cmd/asm/internal/arch: add package definition

The package arch didn't have a definition as you can see in https://tip.golang.org/pkg/cmd/asm/internal/arch/

Change-Id: I07653b396393a75c445d04dbae5e22e90a0d5133
GitHub-Last-Rev: a859e9410f38073853687b933f53eb6570af3216
GitHub-Pull-Request: golang/go#26817
Reviewed-on: https://go-review.googlesource.com/127929
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/asm/internal/arch/arch.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.
 
+// Package arch defines architecture-specific information and support functions.
 package arch
 
 import (