runtime: don't assume AT_RANDOM provide 4-byte aligned ptr
runtime: don't assume AT_RANDOM provide 4-byte aligned ptr

R=dave, remyoudompheng
CC=golang-dev
http://codereview.appspot.com/6854056
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go