liblink: rewrite '\\' in paths to '/' on windows At least three Go tests rely on that (log, runtime/{pprof,debug}).
liblink: rewrite '\\' in paths to '/' on windows
At least three Go tests rely on that (log, runtime/{pprof,debug}).

Fixes issue 6972.
Fixes issue 6974.
Fixes issue 6975.

R=alex.brainman, mattn.jp, rsc
CC=golang-dev
https://codereview.appspot.com/43150043

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