[8u] RFR (XS) 8231398: Add time tracing for gc log rotation at safepoint cleanup

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8231398

There is a nice gotcha when dealing with low latency workloads. The safepoint cleanup does lots of
things, notably rotating the GC logs in 8 (fixed by transition to Unified Logging in 9+, see
JDK-8145092). Those things are normally caught by -XX:+TraceSafepointCleanupTime, but not gc log
rotation, since it misses the tracing statement.

We should consider adding the tracing there.

Webrev:
  https://cr.openjdk.java.net/~shade/8231398/webrev.01/

Testing: eyeballing safepoint cleanup tracing, tier1 (running)

-- 
Thanks,
-Aleksey