Use global TLS variable in proxying.c. NFC

Also use `bool` rather than `int` to better convey intent.

Split out from emscripten-core#24565

Author: sbc100

system/lib/pthread/proxying.c

@@ -106,6 +106,8 @@ static em_task_queue* get_or_add_tasks_for_thread(em_proxying_queue* q,
   return tasks;
 }
 
+static _Thread_local bool executing_system_queue = false;
+
 void emscripten_proxy_execute_queue(em_proxying_queue* q) {
   assert(q != NULL);
   assert(pthread_self());
@@ -114,13 +116,12 @@ void emscripten_proxy_execute_queue(em_proxying_queue* q) {
   // pthread_lock call below that executes the system queue. The per-task_queue
   // recursion lock can't catch these recursions because it can only be checked
   // after the lock has been acquired.
-  static _Thread_local int executing_system_queue = 0;
-  int is_system_queue = q == &system_proxying_queue;
+  bool is_system_queue = q == &system_proxying_queue;
   if (is_system_queue) {
     if (executing_system_queue) {
       return;
     }
-    executing_system_queue = 1;
+    executing_system_queue = true;
   }
 
   pthread_mutex_lock(&q->mutex);
@@ -133,7 +134,7 @@ void emscripten_proxy_execute_queue(em_proxying_queue* q) {
   }
 
   if (is_system_queue) {
-    executing_system_queue = 0;
+    executing_system_queue = false;
   }
 }