sync: restore GOMAXPROCS during benchmarks
sync: restore GOMAXPROCS during benchmarks

R=golang-dev
CC=dvyukov, golang-dev
http://codereview.appspot.com/4631060

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang