sync/atomic: disable store and load test on a single processor machine
sync/atomic: disable store and load test on a single processor machine

Fixes issue 3226.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5756073

dsriram/golang,gnanderson/go,gnanderson/go,gnanderson/go,gnanderson/go,tv42/old-go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,gnanderson/go,tv42/old-go,dsriram/golang,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,dsriram/golang,apparentlymart/golang,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang