runtime: prepare for M's running w/o mcache Can not happen ATM. In preparation for the new scheduler.
runtime: prepare for M's running w/o mcache
Can not happen ATM. In preparation for the new scheduler.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7388043

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