lib9: use $PWD in getwd

lib9: use $PWD in getwd

This makes the full file paths recorded by 6g
prefer $PWD over the actual directory name
(relevant when $PWD gets to the current directory
via symlinks).  It's what everyone else does, and
what people expect.

R=iant, r, mattn.jp
CC=golang-dev
https://golang.org/cl/4824041