ld: dwarf emit filenames in debug_line header instead of as extended opcodes.
ld: dwarf emit filenames in debug_line header instead of as extended opcodes.

Makes it possible for older tools like objdump to find the filenames,
fixes  objdump -d -l --start-address=0x400c00 --stop-address=0x400c36 6.out
fixes issue 1950

R=rsc
CC=golang-dev
http://codereview.appspot.com/4609043

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