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

[Roman Kennke]

>> 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

I did a straight hg qimport and those files did not conflict or
anything, how would that have happened? The orginal patch includes the
same trailing newline removals:

http://hg.openjdk.java.net/jdk/jdk/rev/4ad404da0088

Roman