RFR(s): 8198369: Clean up GCId and GCIdMark

[Per Liden]

After discussing "8198267: Allow GCId::current_raw() calls from 
non-NamedThreads", we noticed the need to do some additional cleanups of 
GCId and GCIdMark. So here's a patch for that.

In short, GCIdMarkAndRestore is not really needed, we can let the normal 
GCIdMark have the restore semantics and get rid of GCIdMarkAndRestore. 
GCId and GCIdMark can also be cleaned up a bit, for example. the 
GCIdMark::_gc_id member is unnecessary, some functions return "const 
uint" instead of "uint", etc.

Bug: https://bugs.openjdk.java.net/browse/JDK-8198369
Webrev: http://cr.openjdk.java.net/~pliden/8198369/webrev.0/

/Per