fixing an assert (dotnet#106752)

we are hitting this assert in gc_thread_function

assert ((n_heaps <= heap_number) || !gc_t_join.joined());

during the init stage. this is because the main thread (that does the GC init) is calling change_heap_count which acts like h0's thread while h0's thread can be anywhere before waiting on ee_suspend_event. so we could be in this situation -

1) main thread calls GCHeap::Init and creates h0's GC thread
2) main thread calls gc_heap::change_heap_count and this will change n_heaps to 1, and after there's another join, 
when the main thread and all other heaps' GC threads have joined, it changes gc_t_join.joined_p to true
3) h0's thread runs and sees that n_heaps is 1, and gc_t_join.joined_p is true which triggers the assert.

so this only happens during the init stage on h0's thread.

src/coreclr/gc/gc.cpp

@@ -6999,8 +6999,21 @@ void gc_heap::gc_thread_function ()
 
     while (1)
     {
-        // inactive GC threads may observe gc_t_join.joined() being true here
-        assert ((n_heaps <= heap_number) || !gc_t_join.joined());
+#ifdef DYNAMIC_HEAP_COUNT
+        if (gc_heap::dynamic_adaptation_mode == dynamic_adaptation_to_application_sizes)
+        {
+            // Inactive GC threads may observe gc_t_join.joined() being true here.
+            // Before the 1st GC happens, h0's GC thread can also observe gc_t_join.joined() being true because it's
+            // also inactive as the main thread (that inits the GC) will act as h0 (to call change_heap_count).
+            assert (((heap_number == 0) && (VolatileLoadWithoutBarrier (&settings.gc_index) == 0)) ||
+                    (n_heaps <= heap_number) ||
+                    !gc_t_join.joined());
+        }
+        else
+#endif //DYNAMIC_HEAP_COUNT
+        {
+            assert (!gc_t_join.joined());
+        }
 
         if (heap_number == 0)
         {