Make System::Layer::mHandleSelectThread atomic.

mHandleSelectThread is used for an optimization: when it's set to the
current thread id, WakeIOThread knows that:

1) We're in the middle of running Layer::HandleTimeout
2) We're doing that on the same thread where WakeIOThread was called

and hence WakeIOThread doesn't need to do anything, because we're
already on the IO thread and it's already awake.

The read of this member in WakeIOThread is not synchronized in any
way, but as long as we guarantee that it correctly reads a value that
was actually assigned to the member (which std::atomic does), things
will work correctly.  Either the value we read will be equal to the
current thread id, in which case we know we're on the one thread that
called Layer::HandleTimeout and are inside that function, or it will
not be equal to our thread id and then we need to do the actual wakeup
work, whatever that value is (including if it's null).

Fixes project-chip#7818

src/system/SystemLayer.h

@@ -41,6 +41,7 @@
 #endif // CHIP_SYSTEM_CONFIG_USE_SOCKETS
 
 #if CHIP_SYSTEM_CONFIG_POSIX_LOCKING
+#include <atomic>
 #include <pthread.h>
 #endif // CHIP_SYSTEM_CONFIG_POSIX_LOCKING
 
@@ -204,7 +205,7 @@ class DLL_EXPORT Layer
     WatchableEventManager mWatchableEvents;
     WakeEvent mWakeEvent;
 #if CHIP_SYSTEM_CONFIG_POSIX_LOCKING
-    pthread_t mHandleSelectThread;
+    std::atomic<pthread_t> mHandleSelectThread;
 #endif // CHIP_SYSTEM_CONFIG_POSIX_LOCKING
 #endif // CHIP_SYSTEM_CONFIG_USE_SOCKETS || CHIP_SYSTEM_CONFIG_USE_NETWORK_FRAMEWORK