Add the gc tag to the logging framework

For JEP-271 we will need the gc tag in the logging framework. To simplify test changes it is good if this tag gets pushed before the actual log changes.

This enables command lines like this to work:

-XX:+IgnoreUnrecognizedVMOptions -XX:+PrintGCDetails -Xlog:gc*

This command line is useful to make tests work both with old and new builds when the changes for JEP-271 are pushed.