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

[Aleksey Shipilev]

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}

-- 
Thanks,
-Aleksey