gc.c (objspace_xcalloc): fix GC accounting

This hopefully works on all platforms with malloc_usable_size.
This may also trigger bugs in places which did not expect GC, too;
so maybe some existing code will need RB_GC_GUARD.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45347 b2dd03c8-39d4-4d8f-98ff-823fe69b080e