Don't enable GC.auto_compact in EnvUtil.under_gc_compact_stress when not supported

tool/lib/envutil.rb

@@ -259,11 +259,15 @@ module EnvUtil
 
   def under_gc_compact_stress(val = :empty, &block)
     raise "compaction doesn't work well on s390x. Omit the test in the caller." if RUBY_PLATFORM =~ /s390x/ # https://github.com/ruby/ruby/pull/5077
-    auto_compact = GC.auto_compact
-    GC.auto_compact = val
+
+    if GC.respond_to?(:auto_compact)
+      auto_compact = GC.auto_compact
+      GC.auto_compact = val
+    end
+
     under_gc_stress(&block)
   ensure
-    GC.auto_compact = auto_compact
+    GC.auto_compact = auto_compact if GC.respond_to?(:auto_compact)
   end
   module_function :under_gc_compact_stress