RFR [8], 2018-07-16, Bulk backports to sh/jdk8

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20180716/webrev.01/

This backports latest work to sh/jdk8:

 [backport] Micro-optimize AArch64 assembly write-barriers
 [backport] Remove C2 write-barrier from .ad files
 [backport] Assembler write barriers should consistently check for forwarded objects
 [backport] Exponential backoff with pacing
 [backport] More detailed pacing histogram
 [backport] Proper units for allocation failure messages
 [backport] Add task termination and enhanced task queue state tracking + weakrefs
 [backport] Report actual free size in non-verbose FreeSet status
 [backport] Pacer for evacuation should print "Avail" to capture discounting
 [backport] Refactor allocation path to accept ShenandoahAllocRequest tuple
 [backport] Elastic TLABs support for Shenandoah
 [backport] Pacer should account actual size for elastic TLABs
 [backport] Heap region count selection should only consider max heap size
 [backport] Fix CAS-obj predicates and add expected-null-versions for cmpxchg-narrow-oop
 [backport] Hook up GCLABs to Elastic LAB support
 [backport] Allocation tracker should really report bytes
 [backport] Complete liveness for recently allocated regions outside the allocation path
 [backport] GCLAB slowpath allocations should fit the object into GCLAB
 [backport] Fix aarch64 CAS predicates
 [backport] Print task queue statistics at the end of GC cycle


Testing: hotspot_tier3_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey