stop the 5c optimizer from clobbering extern static registers.
stop the 5c optimizer from clobbering extern static registers.

R=rsc, ken2
CC=golang-dev
http://codereview.appspot.com/204064

d0f/go-zh,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,d0f/go-zh,d0f/go-zh,npe9/noahevans-go9,Triskite/willstone-goclone,npe9/noahevans-go9,scirelli/scirelli-go,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,rdp/rogerpack2005-golang,webfd/go-zh,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,webfd/go-zh,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,npe9/noahevans-go9,mhennings/marcohennings-go,bryanxu/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,scirelli/scirelli-go,scirelli/scirelli-go,scirelli/scirelli-go,npe9/noahevans-go9,sanjosh/sanjos100-tipc,npe9/noahevans-go9,bryanxu/go-zh,d0f/go-zh,rflanagan/reginaldflanagan-project1,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,rdp/rogerpack2005-golang,mhennings/marcohennings-go,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,webfd/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,npe9/noahevans-go9,scirelli/scirelli-go,Triskite/willstone-goclone,rdp/rogerpack2005-golang,d0f/go-zh,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,npe9/noahevans-go9,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,d0f/go-zh,npe9/noahevans-go9,bryanxu/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,bryanxu/go-zh