runtime: don't build test files that use SetCgoTraceback for gccgo

runtime: don't build test files that use SetCgoTraceback for gccgo

    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