Remove atomics from linked list of blocks

[copybara-service]

Remove atomics from linked list of blocks

We no longer need to traverse the linked list of blocks to check allocated space, which means we also no longer need atomics in the linked list or even its head. This is especially beneficial as the previous implementation contained a race where we could dereference uninitialized memory; because the setting of the next pointers did not use release semantics and the reading of them in SpaceAllocated reads with relaxed order, there's no guarantee that size has actually been initialized - but worse, there is also no guarantee that next has been!. Simplified:

AddBlock:
1 ptr = malloc();
2 ptr->size = 123;
3 ptr->next = ai->blocks;
4 ai->blocks = ptr (release order);

SpaceAllocated:
5 block = ai->blocks (relaxed order)
6 block->size (acquire, but probably by accident)
7 block = block->next (relaxed order)

So I think a second thread calling SpaceAllocated could see the order 1, 4, 5, 6, 7, 2, 3 and read uninitialized memory - there is no data-dependency relationship or happens-before edge that this order violates, and so it would be valid for a compiler+hardware to produce.

In reality, operation 4 will produce an stlr on arm (forcing an order of 1, 2, 3 before 4), and block->next has a data dependency on ai->blocks which would force an ordering in the hardware between 5->6 and 5->7 even for regular ldr instructions.

Delete arena contains, it's private and the only user is its own test.