RFR: Merge conditional GC compilation from upstream

[Aleksey Shipilev]

On 06/06/2018 09:36 AM, Roman Kennke wrote:
> 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?

Looks very good, go!

-Aleksey