RFR: Merge from upstream jdk/jdk10

[Roman Kennke]

I've got the merge from upstream jdk/jdk10 ready.
The most notable stuff:
- new GC interface (e.g. new shenandoahArguments.hpp/cpp classes,
GC-serviceability stuff moved under gc/$GC dirs, lots of refactorings)
- BarrierSet cleanup and new interface for runtime heap access
(BarrierSet::AccessBarrier and friends)

Should considerably reduce Shenandoah-specific shared code changes.

Testing: passes hotspot_gc_shenandoah (release+fastdebug) and sanity
testing with specjvm

A webrev wouldn't be very useful. It brings shenandoah/jdk10 up to the
current revision of jdk/jdk10.

Ok to push?

Roman