The gofrontend doesn't support the runtime.SetCgoTraceback function,
which is specifically for handling mixed Go and C tracebacks.
Use a build tag to avoid compiling the runtime/testdata/testprogcgo
files that refer to SetCgoTraceback. These files are not currently
compiled anyhow, but they will be with a future gotools patch.
Reviewed-on: https://go-review.googlesource.com/46452
From-SVN: r249556
