Split GENCGC_PAGE_BYTES in 3 constants

 * We still have BACKEND_PAGE_BYTES, which should be a conservative
   over-approximation of the OS's page size.

 * GENCGC_PAGE_BYTES is mostly replaced by GENCGC_CARD_BYTES, the
   byte granularity at which we track writes to old generations.
   While we're using mprotect-based write barriers, this should be
   a multiple of BACKEND_PAGE_BYTES.

 * GENCGC_ALLOC_GRANULARITY is the minimum size we attempt to make
   each allocation region.  Should be a multiple of GENCGC_CARD_BYTES.
   While we have largeish cards, this shouldn't ever be an issue, but
   it might if we ever go down to < 1k cards.

 * GENCGC_RELEASE_GRANULARITY is the minimum size at which we release
   address space to the OS.  This should always be a multiple of
   BACKEND_PAGE_BYTES.

 * For now, all three new constants are equal to BACKEND_PAGE_BYTES.