misc/cgo/test: add cgo build constraints

misc/cgo/test: add cgo build constraints

We're about to move this package to cmd/cgo/internal, where it will
get caught up in the "CGO_ENABLED=0 go install cmd" done by make.bash.
Currently, building this package with CGO_ENABLED=0 fails because it
contains several source files that don't themselves import "C", but do
import a subdirectory where that package imports "C" and thus has no
exported API.

Fix the CGO_ENABLED=0 build of this package by adding the necessary
cgo build tags. Not all source files need it, but this CL makes
"CGO_ENABLED=0 go test -c" work in this package.

For #37486.

Change-Id: Id137cdfbdd950eea802413536d990ab642ebcd7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/493215
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Austin Clements <austin@google.com>