Mark sweep free list allocates high address cells first

[qinsoon]

When we allocate a new block or sweep a block, the free list for the block points to the last cell.

mmtk-core/src/util/alloc/free_list_allocator.rs

Line 360 in 6cae51c

block.store_free_list(final_cell);

mmtk-core/src/policy/marksweepspace/native_ms/block.rs

Line 305 in 6cae51c

self.store_free_list(last);

E.g. for a block starting at 0x10000 and ending at 0x20000, with the cell size of 8, the free list is like below:
0x1fff8 -> 0x1fff0 -> 0x1ffe8 -> ... -> 0x10000

So when we allocate, we will get the high address cell first, then the low address cell. I am not sure if this would cause any issue. But clearly we cannot assume that in two consecutively allocated objects, the second object has a higher address than the first object.