runtime: reset parfor stats Otherwise they sum up between GCs.
runtime: reset parfor stats
Otherwise they sum up between GCs.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6535048

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