RFR: Fix x86_32 build

[Aleksey Shipilev]

Need to stub out ShenandoahBarrierSetAssembler::generate_shenandoah_wb for !_LP64:

diff -r e1d9e80ecf2c src/hotspot/cpu/x86/gc/shenandoah/shenandoahBarrierSetAssembler_x86.cpp
--- a/src/hotspot/cpu/x86/gc/shenandoah/shenandoahBarrierSetAssembler_x86.cpp	Fri Jun 15 14:17:43
2018 +0200
+++ b/src/hotspot/cpu/x86/gc/shenandoah/shenandoahBarrierSetAssembler_x86.cpp	Fri Jun 15 16:00:36
2018 +0200
@@ -859,6 +859,7 @@
   StubCodeMark mark(cgen, "StubRoutines", "shenandoah_wb");
   address start = __ pc();

+#ifdef _LP64
   Label not_done;

   // We use RDI, which also serves as argument register for slow call.
@@ -940,7 +941,9 @@
     __ pop(rdi);
   }
   __ ret(0);
-
+#else
+  ShouldNotReachHere();
+#endif
   return start;
 }


Thanks,
-Aleksey