RFR: Merge conditional GC compilation from upstream

[Roman Kennke]

This brings in:

changeset:   49982:9042ffe5b7fe
user:        stefank
date:        Fri May 04 11:41:35 2018 +0200
summary:     8200729: Conditional compilation of GCs

and adds the basic infra for Shenandoah.

By default, Shenandoah is enabled. It can (theoretically) be disabled by
--with-jvm-features=-shenandoahgc but that doesn't work yet. While we've
done a pretty good job at isolating it for, e.g., minimal build, we
still have some leftovers that now need to be excluded by #if
INCLUDE_SHENANDOAHGC. Especially C2 is a bit of a mess, and I don't dare
to touch it. I suggest to go over it and fix it after I pushed this merge.

Testing: tier3_gc_shenandoah good except sporadic failure of
gc/shenandoah/oom/TestClassLoaderLeak.java which seems unrelated.

http://cr.openjdk.java.net/~rkennke/cond-comp-shenandoah/webrev.00/

Ok to push?

Roman