Allocate at least gencgc_alloc_granularity bytes at a time, if possible
* While we have large cards, it doesn't really matter, but tuning that
might help some applications that cons a lot of medium-sized objects.
* Defaults to GENCGC_ALLOC_GRANULARITY, and should always be a multiple
* GENCGC_ALLOC_GRANULARITY defaults to BACKEND_PAGE_BYTES, so this patch
is a no-op.