Fix data race (TSan report) on worker thread startup on macOS in `ire…

…e_thread_request_affinity` (iree-org#15499)

TSan report:

```
WARNING: ThreadSanitizer: data race (pid=45817)
  Read of size 4 at 0x0001084004e0 by thread T2:
    #0 iree_thread_request_affinity threading_darwin.c:230 (local-task_vmvx_semaphore_submission_test:arm64+0x100078f40)
    #1 iree_task_worker_main worker.c:385 (local-task_vmvx_semaphore_submission_test:arm64+0x100071594)
    #2 iree_thread_start_routine threading_darwin.c:72 (local-task_vmvx_semaphore_submission_test:arm64+0x100078e3c)

  Previous write of size 4 at 0x0001084004e0 by main thread:
    #0 iree_thread_create threading_darwin.c:140 (local-task_vmvx_semaphore_submission_test:arm64+0x100078ca4)
    #1 iree_task_worker_initialize worker.c:66 (local-task_vmvx_semaphore_submission_test:arm64+0x1000714f8)
    #2 iree_task_executor_create executor.c:161 (local-task_vmvx_semaphore_submission_test:arm64+0x10006b2b0)
```

The read of `thread->mach_port` at
https://github.com/openxla/iree/blob/ccc4c3719cea467477a783f1c9e9f1fc06b4c508/runtime/src/iree/base/internal/threading_darwin.c#L230

is not ordered relatively to the write of that variable in the parent
thread after `pthread_mach_thread_np` returns:
https://github.com/openxla/iree/blob/ccc4c3719cea467477a783f1c9e9f1fc06b4c508/runtime/src/iree/base/internal/threading_darwin.c#L140

The proposed fix is that the worker thread shouldn't need to access its
own `thread->mach_port`, it can equivalently call `mach_task_self()`.

runtime/src/iree/base/internal/threading_darwin.c

@@ -227,7 +227,7 @@ void iree_thread_request_affinity(iree_thread_t* thread,
   // https://fergofrog.com/code/cbowser/xnu/osfmk/mach/thread_policy.h.html
   // http://www.hybridkernel.com/2015/01/18/binding_threads_to_cores_osx.html
   thread_affinity_policy_data_t policy_data = {affinity.id};
-  thread_policy_set(thread->mach_port, THREAD_AFFINITY_POLICY,
+  thread_policy_set(mach_task_self(), THREAD_AFFINITY_POLICY,
                     (thread_policy_t)(&policy_data),
                     THREAD_AFFINITY_POLICY_COUNT);