RFR (11u): 8224187: Refactor arraycopy_prologue to allow ZGC read barriers on arraycopy

[Roman Kennke]

I'd like to backport the following aarch64-specific change to 11u:

Bug:
https://bugs.openjdk.java.net/browse/JDK-8224187

I need it for recent changes to Shenandoah GC's arraycopy
implementation, namely:

https://bugs.openjdk.java.net/browse/JDK-8231086

The patch only differs from the original patch in that it omits the
Shenandoah parts, which are not in 11u, yet. Other than those, it
applies cleanly.

Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8224187/webrev.00/

Testing: tier1 and tier2 tests, hotspot_gc_shenandoah tests in private
repo that combines this change with the corresponding Shenandoah changes.

Can I please get a review?

Thanks,
Roman