Revert "crypto/elliptic: implement P256 for arm64"

This reverts commit 0246915fbf.

Reason for revert: Broke darwin/arm64 builds.

Change-Id: Iead935d345c4776c0f823f4c152e02bdda308401
Reviewed-on: https://go-review.googlesource.com/121375
Reviewed-by: Andrew Bonventre <andybons@golang.org>

src/crypto/elliptic/p256.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 !amd64,!arm64
+// +build !amd64
 
 package elliptic
 

src/crypto/elliptic/p256_amd64.go

@@ -10,7 +10,7 @@
 // https://link.springer.com/article/10.1007%2Fs13389-014-0090-x
 // https://eprint.iacr.org/2013/816.pdf
 
-// +build amd64 arm64
+// +build amd64
 
 package elliptic
 

src/crypto/elliptic/p256_generic.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 !amd64,!s390x,!arm64
+// +build !amd64,!s390x
 
 package elliptic