Create constant metadata for constant PMCs in default PMC.
Create constant metadata for constant PMCs in default PMC.

This is required to preserve GC invariant when "constant" objects are not marked.

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