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/10083
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/zsyscall_linux_arm64.go

@@ -1,6 +1,8 @@
 // mksyscall.pl syscall_linux.go syscall_linux_arm64.go
 // MACHINE GENERATED BY THE COMMAND ABOVE; DO NOT EDIT
 
+// +build arm64
+
 package unix
 
 import (