Skip test_exivar_resize_with_compaction_stress on s390x

Author: jhawthorn

test/ruby/test_variable.rb

@@ -408,6 +408,7 @@ def test_external_ivars
   end
 
   def test_exivar_resize_with_compaction_stress
+    omit "compaction doesn't work well on s390x" if RUBY_PLATFORM =~ /s390x/ # https://github.com/ruby/ruby/pull/5077
     objs = 10_000.times.map do
       ExIvar.new
     end