cmd/yacc: fix stderr on Windows. Fixes issue 6620.
cmd/yacc: fix stderr on Windows.
Fixes issue 6620.

R=golang-dev, dave, r
CC=golang-dev
https://codereview.appspot.com/15330043

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