libgo: update to final Go 1.8 release

libgo: update to final Go 1.8 release

    Along with the update this fixes a problem that was always present but
    only showed up with the new reflect test.  When a program used a
    **unsafe.Pointer and stored the value in an interface type, the
    generated type descriptor pointed to the GC data for *unsafe.Pointer.
    It did that by name, but we were not generating a variable with the
    right name.

    Reviewed-on: https://go-review.googlesource.com/37144

From-SVN: r245535