RFR (XS): 8200730: Fix debug=gc+phases time tracking in Remark and Cleanup

[Thomas Schatzl]

Hi all,

  can I have reviews to let the recently introduced timing measurements
in Remark and Cleanup pauses actually show useful numbers?

The problem is that the GCTraceTime instances were not assigned to a
variable inside a scope so the compiler immediately destructed it,
always giving "0.000ms" lengths.

Also added to use the gc timer to these lines for JFR support.

I would like to think this is a trivial change.

CR:
https://bugs.openjdk.java.net/browse/JDK-8200730
Webrev:
http://cr.openjdk.java.net/~tschatzl/8200730/webrev
Testing:
manual verification that the shown numbers are sane

Thanks,
  Thomas