[8] RFR: Move barrier sets to their proper locations

[Roman Kennke]

Ok. I was wondering the same for a while, but always assumed we'd need
build-machinery changes to pick up stuff under subdirectories.

Roman

On Wed, 2020-05-20 at 09:49 +0200, Aleksey Shipilev wrote:
> https://cr.openjdk.java.net/~shade/shenandoah/8u-move-sbs/webrev.01/
> 
> It always bothers me with backports that barrier set files and
> shenandoahSupport is at inconvenient
> places in sh/jdk8. We are better move them to the location that match
> sh/jdk11 and jdk/jdk.
> 
> Testing: Linux x86_64 builds {fastdebug,release}
>