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
http://codereview.appspot.com/4824041

dsriram/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,gnanderson/go,dsriram/golang,dsriram/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,dsriram/golang,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,tv42/old-go,gnanderson/go,gnanderson/go,gnanderson/go