RFR: 8152955: Many safepoints of "no vm operation" kind

[Robbin Ehn]

Hi all, please review.

When cleanup, safepointalot or vm halt starts a safepoint without an operation we now set a reason string for.

Webrev: http://cr.openjdk.java.net/~rehn/8152955/
Bug: https://bugs.openjdk.java.net/browse/JDK-8152955

Passes hotspot_tier1

Thanks, Robbin