don't print task backtrace for GC threads (JuliaLang#51413)

GC threads don't have tasks associated with them.

src/stackwalk.c

@@ -1125,6 +1125,8 @@ JL_DLLEXPORT void jl_print_backtrace(void) JL_NOTSAFEPOINT
  jlbacktrace();
 }
 
+extern int gc_first_tid;
+
 // Print backtraces for all live tasks, for all threads.
 // WARNING: this is dangerous and can crash if used outside of gdb, if
 // all of Julia's threads are not stopped!
@@ -1133,6 +1135,10 @@ JL_DLLEXPORT void jl_print_task_backtraces(int show_done) JL_NOTSAFEPOINT
  size_t nthreads = jl_atomic_load_acquire(&jl_n_threads);
  jl_ptls_t *allstates = jl_atomic_load_relaxed(&jl_all_tls_states);
  for (size_t i = 0; i < nthreads; i++) {
+ // skip GC threads since they don't have tasks
+ if (gc_first_tid <= i && i < gc_first_tid + jl_n_gcthreads) {
+ continue;
+ }
  jl_ptls_t ptls2 = allstates[i];
  arraylist_t *live_tasks = &ptls2->heap.live_tasks;
  size_t n = live_tasks->len;