MINOR: debug: make the mem_stats section aligned to void*

MINOR: debug: make the mem_stats section aligned to void*

Not specifying the alignment will let the linker choose it, and it turns
out that it will not necessarily be the same size as the one chosen for
struct mem_stats, as can be seen if any new fields are added there. Let's
enforce an alignment to void* both for the section and for the structure.