Further improve of linker memory allocators

This includes:
  - Blocks allocated by small object allocators are now aligned to
    their block size.
  - Remove redundant free_blocks_cnt and allocated_blocks_cnt, as they
    sum up to a fixed number.
  - Remove a redundant padding word in small_object_page_info on 32-bit
    platform.

Test: Build and boot cuttlefish.
Change-Id: Ib922d7af739e3709e8a162c7f68e7f99bf95a914