compiler, runtime: support and use single argument go:linkname

compiler, runtime: support and use single argument go:linkname

    The gc compiler has started permitting go:linkname comments with a
    single argument to mean that a function should be externally visible
    outside the package.  Implement this in the Go frontend.

    Change the libgo runtime package to use it, rather than repeating the
    name just to export a function.

    Remove a couple of unnecessary go:linkname comments on declarations.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/192197

From-SVN: r275239