Defer setting gc_stress until inits done

[Bug #15784]

gc.c

@@ -2571,6 +2571,8 @@ Init_heap(void)
     objspace->id_to_obj_tbl = st_init_numtable();
     objspace->obj_to_id_tbl = st_init_numtable();
 
+    dont_gc = 1;
+
     gc_stress_set(objspace, ruby_initial_gc_stress);
 
 #if RGENGC_ESTIMATE_OLDMALLOC
@@ -2584,6 +2586,14 @@ Init_heap(void)
     finalizer_table = st_init_numtable();
 }
 
+void
+Init_gc_stress(void)
+{
+    rb_objspace_t *objspace = &rb_objspace;
+
+    dont_gc = 0;
+}
+
 typedef int each_obj_callback(void *, void *, size_t, void *);
 
 struct each_obj_args {

inits.c

@@ -67,5 +67,6 @@ rb_call_inits(void)
     CALL(vm_trace);
     CALL(vm_stack_canary);
     CALL(ast);
+    CALL(gc_stress);
 }
 #undef CALL

test/ruby/test_gc.rb

@@ -384,6 +384,10 @@ class TestGc < Test::Unit::TestCase
     end;
   end
 
+  def test_gc_stress_at_startup
+    assert_in_out_err([{"RUBY_DEBUG"=>"gc_stress"}], '', [], [], '[Bug #15784]', success: true)
+  end
+
   def test_gc_disabled_start
     begin
       disabled = GC.disable