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

rflanagan/reginaldflanagan-project1,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,d0f/go-zh,rflanagan/reginaldflanagan-project1,d0f/go-zh,mhennings/marcohennings-go,scirelli/scirelli-go,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,bryanxu/go-zh,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,rdp/rogerpack2005-golang,scirelli/scirelli-go,bryanxu/go-zh,d0f/go-zh,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,scirelli/scirelli-go,scirelli/scirelli-go,scirelli/scirelli-go,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,scirelli/scirelli-go,mhennings/marcohennings-go,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,webfd/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,d0f/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,d0f/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,bryanxu/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,d0f/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,d0f/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,bryanxu/go-zh,sanjosh/sanjos100-tipc,scirelli/scirelli-go,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1