Do GMS sanity check only when DETAILED_GC_DEBUG set.
Do GMS sanity check only when DETAILED_GC_DEBUG set.

This speed-up debug builds a lot.

gagern/parrot,parrot/parrot,FROGGS/parrot,youprofit/parrot,parrot/parrot,tewk/parrot-select,youprofit/parrot,gitster/parrot,FROGGS/parrot,tkob/parrot,tewk/parrot-select,FROGGS/parrot,parrot/parrot,youprofit/parrot,gitster/parrot,gitster/parrot,FROGGS/parrot,parrot/parrot,youprofit/parrot,gagern/parrot,FROGGS/parrot,gagern/parrot,tewk/parrot-select,tkob/parrot,tewk/parrot-select,FROGGS/parrot,tewk/parrot-select,youprofit/parrot,tkob/parrot,tewk/parrot-select,gagern/parrot,gitster/parrot,tewk/parrot-select,gitster/parrot,gagern/parrot,youprofit/parrot,youprofit/parrot,tkob/parrot,FROGGS/parrot,gagern/parrot,gitster/parrot,tkob/parrot,tkob/parrot,parrot/parrot,gitster/parrot,youprofit/parrot,tkob/parrot,tkob/parrot,FROGGS/parrot,gagern/parrot