unix: correct build tag to define Getdirentries for darwin/arm with Go 1.11

Fixes golang/go#36300

Change-Id: I42692b47135870b52a26eb9662ca54648c9cfaf2
Reviewed-on: https://go-review.googlesource.com/c/sys/+/212459
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
diff --git a/unix/syscall_darwin_arm.1_11.go b/unix/syscall_darwin_arm.1_11.go
index c81510d..0e3f25a 100644
--- a/unix/syscall_darwin_arm.1_11.go
+++ b/unix/syscall_darwin_arm.1_11.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin,386,!go1.12
+// +build darwin,arm,!go1.12
 
 package unix