build: disable cgo on windows

Update issue 4955.

R=golang-dev
CC=golang-dev
https://codereview.appspot.com/7435049
runtime: fix build on openbsd, plan9

R=golang-dev
CC=golang-dev
https://codereview.appspot.com/7438052
net/http/cgi: disable TestCopyError to try to fix darwin/386 on builder

Update issue 4958.

R=golang-dev
CC=golang-dev
https://codereview.appspot.com/7434047