cmd/go: ignore errors from go/build for standard packages

cmd/go: ignore errors from go/build for standard packages

    The go/build package does not know that gccgo's standard packages don't
    have source, and will report an error saying that it can not find them.
    Work around that in the cmd/go sources, since the go/build sources don't
    currently have a list of standard packages.

    This should get a real fix in the master sources, somehow.

    Fixes golang/go#16701.

    Reviewed-on: https://go-review.googlesource.com/27052

From-SVN: r239486