RFR: Fix Zero build

[Roman Kennke]

Good! Thanks,

Roman

> Zero failed to build, because BarrierSetAssembler is not implemented there, and using it in empty
> subclass would fail to compile. All other BSes have the same form as below:
> 
> diff -r d3e906f125ba src/hotspot/cpu/zero/gc/shenandoah/shenandoahBarrierSetAssembler_zero.hpp
> --- a/src/hotspot/cpu/zero/gc/shenandoah/shenandoahBarrierSetAssembler_zero.hpp Sat Apr 28 18:53:03
> 2018 +0200
> +++ b/src/hotspot/cpu/zero/gc/shenandoah/shenandoahBarrierSetAssembler_zero.hpp Sat Apr 28 19:22:05
> 2018 +0200
> @@ -27,7 +27,6 @@
>  #include "asm/macroAssembler.hpp"
>  #include "gc/shared/barrierSetAssembler.hpp"
> 
> -class ShenandoahBarrierSetAssembler: public BarrierSetAssembler {
> -};
> +class ShenandoahBarrierSetAssembler;
> 
>  #endif // CPU_ZERO_GC_SHENANDOAH_SHENANDOAHBARRIERSETASSEMBLER_ZERO_HPP
> 
> Testing: Zero x86_64 build
> 
> Thanks,
> -Aleksey
>