runtime: export cgoCheck functions

runtime: export cgoCheck functions

    The functions cgoCheckPointer and cgoCheckResult are called by code
    generated by cgo. That means that we need to export them using
    go:linkname, as otherwise they are local symbols. The cgo code
    currently uses weak references to only call the symbols if they are
    defined, which is why it has been working--the cgo code has not been
    doing any checks.

    Reviewed-on: https://go-review.googlesource.com/80295

From-SVN: r255347