chacha20: drop Go 1.10 compatibility for arm64

Other packages already dropped compatibility with go < 1.12, so it should be safe to remove it for this package as well.

Change-Id: Ib1424763e3aa94d0187a667ebee058100136f53b
GitHub-Last-Rev: 51df9690a5f37ba50d5ae5e84cf31b78fb6c5cd8
GitHub-Pull-Request: golang/crypto#241
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/448241
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Joedian Reid <joedian@golang.org>
diff --git a/chacha20/chacha_arm64.go b/chacha20/chacha_arm64.go
index 94c71ac..5dfacbb 100644
--- a/chacha20/chacha_arm64.go
+++ b/chacha20/chacha_arm64.go
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.11 && gc && !purego
-// +build go1.11,gc,!purego
+//go:build gc && !purego
+// +build gc,!purego
 
 package chacha20
 
diff --git a/chacha20/chacha_arm64.s b/chacha20/chacha_arm64.s
index 63cae9e..f1f6623 100644
--- a/chacha20/chacha_arm64.s
+++ b/chacha20/chacha_arm64.s
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.11 && gc && !purego
-// +build go1.11,gc,!purego
+//go:build gc && !purego
+// +build gc,!purego
 
 #include "textflag.h"
 
diff --git a/chacha20/chacha_noasm.go b/chacha20/chacha_noasm.go
index 025b498..02ff3d0 100644
--- a/chacha20/chacha_noasm.go
+++ b/chacha20/chacha_noasm.go
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build (!arm64 && !s390x && !ppc64le) || (arm64 && !go1.11) || !gc || purego
-// +build !arm64,!s390x,!ppc64le arm64,!go1.11 !gc purego
+//go:build (!arm64 && !s390x && !ppc64le) || !gc || purego
+// +build !arm64,!s390x,!ppc64le !gc purego
 
 package chacha20