[generational_gc] add some casts to get g++ to build
[generational_gc] add some casts to get g++ to build

git-svn-id: 0925b7c4d71351b4fea77b99f2cfcd1a0170cafc@49674 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

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