crypto/sha1: restore accidental removal of arm build tag

Regression from https://golang.org/cl/68490043/diff/60001/src/pkg/crypto/sha1/sha1block_decl.go

Fixes arm build.

TBR=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/68080046

src/pkg/crypto/sha1/sha1block_decl.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 amd64p32 386
+// +build amd64 amd64p32 arm 386
 
 package sha1