unix: Make the MICROPY_xxx_ATOMIC_SECTION mutex recursive.

This mutex is used to make the unix port behave more like bare metal, i.e.
it allows "IRQ handlers" to run exclusively by making the mutex recursive.

ports/unix/btstack_usb.c

@@ -156,7 +156,10 @@ STATIC void *btstack_thread(void *arg) {
     // Or, if a timeout results in it being set to TIMEOUT.
 
     while (mp_bluetooth_btstack_state == MP_BLUETOOTH_BTSTACK_STATE_STARTING || mp_bluetooth_btstack_state == MP_BLUETOOTH_BTSTACK_STATE_ACTIVE) {
+        // Pretend like we're running in IRQ context (i.e. other things can't be running at the same time).
+        mp_uint_t atomic_state = MICROPY_BEGIN_ATOMIC_SECTION();
         btstack_run_loop_embedded_execute_once();
+        MICROPY_END_ATOMIC_SECTION(atomic_state);
 
         // The USB transport schedules events to the run loop at 1ms intervals,
         // and the implementation currently polls rather than selects.

ports/unix/mpthreadport.c

@@ -65,7 +65,7 @@ STATIC pthread_key_t tls_key;
 // The mutex is used for any code in this port that needs to be thread safe.
 // Specifically for thread management, access to the linked list is one example.
 // But also, e.g. scheduler state.
-STATIC pthread_mutex_t thread_mutex = PTHREAD_MUTEX_INITIALIZER;
+STATIC pthread_mutex_t thread_mutex;
 STATIC thread_t *thread;
 
 // this is used to synchronise the signal handler of the thread
@@ -111,6 +111,13 @@ void mp_thread_init(void) {
     pthread_key_create(&tls_key, NULL);
     pthread_setspecific(tls_key, &mp_state_ctx.thread);
 
+    // Needs to be a recursive mutex to emulate the behavior of
+    // BEGIN_ATOMIC_SECTION on bare metal.
+    pthread_mutexattr_t thread_mutex_attr;
+    pthread_mutexattr_init(&thread_mutex_attr);
+    pthread_mutexattr_settype(&thread_mutex_attr, PTHREAD_MUTEX_RECURSIVE);
+    pthread_mutex_init(&thread_mutex, &thread_mutex_attr);
+
     // create first entry in linked list of all threads
     thread = malloc(sizeof(thread_t));
     thread->id = pthread_self();