Backport reshaped write barrier expansion to JDK8u

[Roman Kennke]

This backports the new reshaped WB expansion to JDK8u, plus all sorts of
support code, etc. The idea is to make shenandoahSupport.* look like its
counterpart in sh/jdk as much as possible. Changes stuff like 'new
BlahNode()' -> 'new (C) BlahNode()', leaves out LSM and some minor
diversions, but mostly it's like in sh/jdk now. It's also still lacking
the post-expand-optimization-pass, I'll look into porting that in one of
the next patches.

This passes hotspot_gc_shenandoah (release/fastdebug) and a bunch of
specjvm point tests.

I'll wait with this patch until we re-initialized sh/jdk8u from
aarch64/jdk8u-sh. But can I get a review in the meantime?

http://cr.openjdk.java.net/~rkennke/shjdk8-wb-expansion/webrev.00/

Roman