RFR: Cherry-pick: 8213199: GC abstraction for Assembler::needs_explicit_null_check() plus Shenandoah parts

[Aleksey Shipilev]

On 11/12/2018 10:36 PM, Roman Kennke wrote:
> In preparation for next upstream merge (reduce conflicts and headaches
> sorting it out), this cherry picks "8213199: GC abstraction for
> Assembler::needs_explicit_null_check()" plus the corresponding Zero
> fixes by Aleksey (JDK-8213711) and adds the Shenandoah parts.
> 
> http://cr.openjdk.java.net/~rkennke/JDK-8213199-shenandoah/webrev.00/

OK.

I think there are trailing newline differences in:
  src/hotspot/cpu/arm/macroAssembler_arm.hpp
  src/hotspot/os_cpu/aix_ppc/os_aix_ppc.cpp
  src/hotspot/os_cpu/linux_arm/os_linux_arm.cpp

-Aleksey