RFR: 8224187 Refactor arraycopy_prologue to allow ZGC read barriers on arraycopy

[Stuart Monteith]

Hello,
   On aarch64 the signature for
barrierSetAssembler::arraycopy_prologue needs to be changed in order
accept the source as well as the destination array for ZGC's barriers.

The bug:
 https://bugs.openjdk.java.net/browse/JDK-8224187
and the patch:
  http://cr.openjdk.java.net/~smonteith/8224187/webrev.0/


BR,
   Stuart