RFR: Fix x86_32 build

[Aleksey Shipilev]

There is a simple signature mismatch on !_LP64 path this breaks x86_32 build for
sh/jdk. Fixed like this:

> diff -r e71660c01da7 src/hotspot/cpu/x86/gc/shenandoah/shenandoahBarrierSetAssembler_x86.cpp
> --- a/src/hotspot/cpu/x86/gc/shenandoah/shenandoahBarrierSetAssembler_x86.cpp   Thu Sep 13 10:19:54 2018 +0200
> +++ b/src/hotspot/cpu/x86/gc/shenandoah/shenandoahBarrierSetAssembler_x86.cpp   Thu Sep 13 13:12:45 2018 +0200
> @@ -595,5 +595,5 @@
>  // due to concurrent evacuation.
>  #ifndef _LP64
> -void ShenandoahBarrierSetAssembler::cmpxchg_oop(MacroAssembler* masm, DecoratorSet decorators,
> +void ShenandoahBarrierSetAssembler::cmpxchg_oop(MacroAssembler* masm,
>                                                  Register res, Address addr, Register oldval, Register newval,
>                                                  bool exchange, bool encode, Register tmp1, Register tmp2) {

Testing: x86_32 cross-build

Thanks,
-Aleksey