RFR [11u]: Bulk backports to sh/jdk11

[Aleksey Shipilev]

Let's pick up a few easy backports to sh/jdk11:
 [backport] 8222125: Shenandoah: Crash when running with ShenandoahParallelSafepointThreads=1
 [backport] 8222130: Shenandoah should verify roots after pre-evacuation
 [backport] 8222185: Shenandoah should report "committed" as capacity
 [backport] 8222186: Shenandoah should not uncommit below minimum heap size
 [backport] 8221735: Shenandoah fails ctw/modules/jdk_management_agent.java with Traversal
 [backport] 8215356: Disable x86_32 Shenandoah build to avoid hotspot/tier1 failures

Webrev:
  http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk11-20190424/webrev.01/

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey