RFR: sh/jdk8 backport of JDK-8217016: Shenandoah: Streamline generation of CAS barriers

[Roman Kennke]

Now that we re-seeded sh/jdk8, we can finally resume backporting jdk8
fixes. :-)

This backports the streamlining of CAS barriers and related fix to jdk8.

Since much of it was shuffling around of the GC interfaces, which we
don't have in 8, I focused on getting the actual implementation to be
the same as in 11.

Curiously, there was nothing to do for x86. We probably ran in circle
with the cmpxchg_oop_shenandoah stuff :-)

The aarch64 cmpxchg_oop_shenandoah is now equivalent to the jdk11 impl.

http://cr.openjdk.java.net/~rkennke/backport-jdk8-2019-05-14/webrev.00/

Testing: hotspot_gc_shenandoah ok

Good to push?

Roman