compiler: implement go:linkname compiler directive

compiler: implement go:linkname compiler directive

    Implement the go:linkname compiler directive for functions (both
    function definitions and function declarations).  At least for now, give
    an error for code that tries to use go:linkname with a non-function.

    Reviewed-on: https://go-review.googlesource.com/26651

From-SVN: r239311