runtime: disable aeshash on NaCl at compile time
runtime: disable aeshash on NaCl at compile time

Fixes build for nacl/386.

LGTM=dave
R=khr, bradfitz, dave, dan.kortschak, rsc
CC=golang-codereviews
https://codereview.appspot.com/121080043

sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh