Memory pool: compare block addresses without overflow

Memory pool: compare block addresses without overflow

Previously, the rbtree comparator converted the difference between two
addresses to intptr_t.  On 32-bit systems, addresses that differ by at
least 2 GiB could produce an incorrect ordering and prevent a later block
lookup from finding its cluster.

The fix is to compare the addresses directly and return -1, 0, or 1.
This fixes a pool that remained non-empty after all allocations were freed
in the x86 memory pool test.