More conservative defaults in GENCGC

More conservative defaults in GENCGC

 * Set GENCGC-ALLOC-GRANULARITY to 0, and thus fully disable the minimal
   allocation size logic by default.

 * Explicitly zero out pages that fall outside GENCGC-RELEASE-GRANULARITY,
   instead of leaving them dirty, when releasing address space.

 * Set large_object_size to 4 * MAX(PAGE_SIZE, GENCGC_ALLOC_GRANULARITY,
   GENCGC_RELEASE_GRANULARITY).

 * I'm getting very strange failures in some branches that I can't replicate
   on master, but let's be extra careful for now.