RFR: Fix Zero build

[Aleksey Shipilev]

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