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

firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old