unix: add arm64 and ppc64le build tags as needed for Go 1.4

Go1.4 does not recognize arm64 and ppc64le as valid GOARCH values, so
we need explicit build tags.

Change-Id: I1a886c132ae398d9f92b2c2b33d2a4827bbdb9d4
Reviewed-on: https://go-review.googlesource.com/10083Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/asm_linux_arm64.s

@@ -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 arm64
 // +build !gccgo
 
 #include "textflag.h"

unix/syscall_linux_arm64.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 arm64
+
 package unix
 
 const _SYS_dup = SYS_DUP3

unix/zerrors_linux_arm64.go

@@ -4,6 +4,8 @@
 // Created by cgo -godefs - DO NOT EDIT
 // cgo -godefs -- _const.go
 
+// +build arm64
+
 package unix
 
 import "syscall"

unix/zerrors_linux_ppc64le.go

@@ -4,6 +4,8 @@
 // Created by cgo -godefs - DO NOT EDIT
 // cgo -godefs -- -m64 _const.go
 
+// +build ppc64le
+
 package unix
 
 import "syscall"

unix/zsyscall_linux_arm64.go

 // mksyscall.pl syscall_linux.go syscall_linux_arm64.go
 // MACHINE GENERATED BY THE COMMAND ABOVE; DO NOT EDIT
 
+// +build arm64
+
 package unix
 
 import (

unix/zsyscall_linux_ppc64le.go

 // mksyscall.pl syscall_linux.go syscall_linux_ppc64x.go
 // MACHINE GENERATED BY THE COMMAND ABOVE; DO NOT EDIT
 
+// +build ppc64le
+
 package unix
 
 import (

unix/zsysnum_linux_arm64.go

 // mksysnum_linux.pl /usr/include/asm-generic/unistd.h
 // MACHINE GENERATED BY THE ABOVE COMMAND; DO NOT EDIT
 
+// +build arm64
+
 package unix
 
 const (

unix/zsysnum_linux_ppc64le.go

 // mksysnum_linux.pl /usr/include/powerpc64le-linux-gnu/asm/unistd.h
 // MACHINE GENERATED BY THE ABOVE COMMAND; DO NOT EDIT
 
+// +build ppc64le
+
 package unix
 
 const (

unix/ztypes_linux_arm64.go

 // Created by cgo -godefs - DO NOT EDIT
 // cgo -godefs -- -fsigned-char types_linux.go
 
+// +build arm64
+
 package unix
 
 const (

unix/ztypes_linux_ppc64le.go

 // Created by cgo -godefs - DO NOT EDIT
 // cgo -godefs types_linux.go
 
+// +build ppc64le
+
 package unix
 
 const (