runtime: improve precision of GC_REGION
runtime: improve precision of GC_REGION

R=rsc
CC=golang-dev
https://codereview.appspot.com/7383054

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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