JDK-8144016: Add the gc tag to the logging framework

[Bengt Rutisson]

Hi everyone,

Could I have a couple of reviews for this very small change?

http://cr.openjdk.java.net/~brutisso/8144016/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8144016

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 change adds the tag but no usages of it yet. 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.

Thanks,
Bengt