[GC] Skipped PMCNULL marking as live in the Parrot_gc_mark_PMC_alive macro for all calls within the core.  This gives a 1.806% performance improvement in the fib.pir benchmark.
[GC] Skipped PMCNULL marking as live in the Parrot_gc_mark_PMC_alive macro for
all calls within the core.  This gives a 1.806% performance improvement in the
fib.pir benchmark.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@42001 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot