RFR(sh/jdk8): [backport] 8228369: Shenandoah: Refactor LRB C1 stubs

[Roman Kennke]

This backports the following issue to jdk8:

https://bugs.openjdk.java.net/browse/JDK-8228369

This is in preparation for the SFX barrier backport.

Due to missing GC interfaces, I had to put some code in c1_Runtime*, it 
is following the same pattern as other GC runtime stubs.

http://cr.openjdk.java.net/~rkennke/JDK-8228369-jdk8/webrev.00/

Let's also include a follow-ups:

https://bugs.openjdk.java.net/browse/JDK-8229709
http://cr.openjdk.java.net/~rkennke/JDK-8229709-jdk8/webrev.00/

The other two follow-ups ( JDK-8229470 and JDK-8229977) don't apply to 
jdk8 because of different implementations.

Can I please get a review?

Thanks,
Roman