cmd/go: add GOOS/GOARCH to go version output
cmd/go: add GOOS/GOARCH to go version output

Fixes issue 4492.

% go version
go version devel +6b602ab487d6 Sat Dec 08 14:43:00 2012 +0100 linux/amd64

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/6906058

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