RFR: Upstream merge jdk/jdk11 -> shenandoah/jdk11 2018-07-27

[Roman Kennke]

This merges in jdk-11+25, and adds corresponding
shenandoah-jdk-11+25 tag. The same 'precise'
merging+tagging procedure as last time was used.

Only minor/trival merge conflicts arose and no significant (to
Shenandoah) new features arrived.

List of stuff merged in:
http://cr.openjdk.java.net/~rkennke/jdk11-upstream-merge-2018-08-07/outgoing.txt

Testing: tier3_gc_shenandoah (ongoing, expected ok before push)

Ok?
Roman