RFR: Factor out keep alive barriers in aarch64

[Aleksey Shipilev]

On 09/08/2017 05:24 PM, Roman Kennke wrote:
> This is the same that I did for x86, but the aarch64 part.
> 
> Test: hotspot_gc_shenandoah
> 
> http://cr.openjdk.java.net/~rkennke/keepaliveaarch64/webrev/
> 
> Ok?

OK.

-Aleksey