RFR: 8186476: Generalize Atomic::add with templates

[Kim Barrett]

> On Aug 20, 2017, at 2:16 AM, Kim Barrett <kim.barrett at oracle.com> wrote:
> 
> Please review this change to Atomic::add, making it a function
> template with a per-platform underlying implementation.
> 
> This change is similar to 8186166: "Generalize Atomic::cmpxchg with
> templates", with similar goals and taking a similar approach.
> 
> […]
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8186476
> 
> Webrev:
> http://cr.openjdk.java.net/~kbarrett/8186476/hotspot.00/
> 
> Testing:
> Ad hoc hotspot nightly test.

I forgot to mention that this change is based on the 8186166 hotspot.04 change,
which has not yet been pushed.  I’m leaving that to Erik, as well as handing this
change off to him.