Apparently, the Windows routines sometimes fail to generate output.
Copy the Unix stdio-based implementations instead.

Suggested by Pietro Gagliardi in CL 65280043 but that CL
seems to have been abandoned.

Fixes #7242.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/71550044