RFR: Consolidate AArch64 assembly write-barriers

[Aleksey Shipilev]

On 07/09/2018 08:04 PM, Roman Kennke wrote:
> This extracts a small part from the 'Consolidate write-barriers' patch.
> 
> It makes the first bunch of gc-state-loads, checks and read-barriers
> consistent in both versions of the write-barrier, and removes the
> membar, which is no longer needed because we don't race with turning
> evacuation off anymore.

Looks good! Suggestion for changeset synopsis: "Micro-optimize AArch64 assembly write-barriers".

-Aleksey