RFR: 9+8u: MacroAssembler and ShenandoahBarrierSet reference x86_64 registers

[Aleksey Shipilev]

Hi,

Another batch of i586 build failure fixes. Conditionalize shared Shenandoah code
with _LP64 where they reference x86_64 registers. This allows us to build:
  http://cr.openjdk.java.net/~shade/shenandoah/build/noregs-i586/webrev.01/

This affects both 9 and 8u.

Testing: hotspot_gc_shenandoah for both 9 and 8u.

-Aleksey