crypto/sha512: avoid duplicate block declaration on 386
crypto/sha512: avoid duplicate block declaration on 386

Unbreak the build - we do not have a sha512 block implementation in
386 assembly (yet).

R=golang-codereviews, dave
CC=golang-codereviews
https://codereview.appspot.com/48520043

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