internal/cpu: remove redundant build tag

The file name suffix arm64 already limits the file to be build only on arm64.

Change-Id: I33db713041b6dec9eb00889bac3b54c727e90743
Reviewed-on: https://go-review.googlesource.com/108986
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/internal/cpu/cpu_arm64.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build arm64
-
 package cpu
 
 const CacheLineSize = 64