Expose PL_dirty, the flag that marks global destruction

Perl's global destruction is a little tricky to deal with with respect to finalizers because it's not ordered and objects can sometimes disappear. Writing defensive destructors is hard and annoying, and usually if global destruction is happening you only need the destructors that free up non process local resources to actually execute. For these constructors you can avoid the mess by simply bailing out if global destruction is in effect.