RFR: 8154106: UL Xlog:help regd'g 'rt' tag

[Robbin Ehn]

Hi all, please review!

java -Xlog:help produces examples which do not work due to the fact the tag 'rt' does not exists.
This small patch changes the examples to use tag 'safepoint' instead, which is not likely to go anywhere.

Bug: https://bugs.openjdk.java.net/browse/JDK-8154106
Webrev: http://cr.openjdk.java.net/~rehn/8154106/webrev/

Thanks!

/Robbin