RFR: aarch64: add support for vectorizing floating-point fabs & fneg

[Roland Westrelin]

> I have created a JIRA issue and webrev for this.
> 
> https://bugs.openjdk.java.net/browse/JDK-8138583
> 
> http://cr.openjdk.java.net/~enevill/8138583/webrev/
> 
> As this modifies shared code could someone sponsor it and push it through JPRT.

I’m pushing it.

Roland.