[Runtime] Align `iree_async_posix_worker_t` (#24589)

Fix #24578

The fix includes aligning `iree_async_posix_worker_t` to
`iree_hardware_constructive_interference_size`.

Pertaining to the concern of possible improper offset via pointer
casting mentioned in this
[comment](https://github.com/iree-org/iree/issues/24578#issuecomment-4646834986)
, it comes from line 179 in file
`runtime\src\iree\async\platform\posix\proactor.c`:
```cpp
  // Set up pointers into trailing data.
  proactor->workers =
      (iree_async_posix_worker_t*)((uint8_t*)proactor + workers_offset); // <- This
  proactor->worker_count = worker_count;
```

The worker offset is calculated from line 148 in the same file:
```cpp
  IREE_RETURN_AND_END_ZONE_IF_ERROR(
      z0, IREE_STRUCT_LAYOUT(
              sizeof(iree_async_proactor_posix_t), &total_size,
              IREE_STRUCT_FIELD_ALIGNED(
                  worker_count, iree_async_posix_worker_t,
                  iree_hardware_destructive_interference_size, &workers_offset), // <- This
              IREE_STRUCT_FIELD_ALIGNED(
                  ready_pool_capacity, iree_async_posix_ready_op_t,
                  iree_hardware_destructive_interference_size,
                  &ready_entries_offset),
              IREE_STRUCT_FIELD(completion_pool_capacity,
                                iree_async_posix_completion_t,
                                &completion_entries_offset),
              IREE_STRUCT_FIELD(message_pool_capacity,
                                iree_async_message_pool_entry_t,
                                &message_entries_offset)))
```

From what I read, `IREE_STRUCT_FIELD_ALIGNED` considers the alignment. I
supposed the offset calculation should be correct, especially with the
alignment to be the same.

Moreover, I've also saw some manual padding in the struct via
`_padding`. I remove such padding as I suppose the alignment will pad
for us.

---------

Signed-off-by: Rechie Kho <50512341+RechieKho@users.noreply.github.com>