cmd/cgo: can't access C.Stdout if stdout is a macro

[4ad]

This affects Windows, NetBSD, OpenBSD and Solaris. Ian suggested cgo should detect this special case and make it work.

[gopherbot]

CL https://golang.org/cl/43970 mentions this issue.

[ALTree]

There's a CL for this but it's unfinished (still marked DO NOT REVIEW) and we're deep in the freeze, so moving this to the go1.10 milestone.