crypto/sha1: restore accidental removal of arm build tag
crypto/sha1: restore accidental removal of arm build tag

Regression from https://codereview.appspot.com/68490043/diff/60001/src/pkg/crypto/sha1/sha1block_decl.go

Fixes arm build.

TBR=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/68080046

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang