[11] RFR: Split Shenandoah C2 rules into separate .ad files

[Aleksey Shipilev]

https://cr.openjdk.java.net/~shade/shenandoah/11u-split-ad/webrev.01/

This does what our later releases do: split Shenandoah rules into separate .ad files. This
alleviates quite a bit of upstream exposure and also makes backports from dev easier. After this
change, there are no differences against upstream in x86_32.ad and x86_64.ad. There are still some
minor leftovers in aarch64.ad. New files are also very close to the versions in jdk/jdk.

Testing: {x86_32, x86_64, aarch64} hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey