PPC/S390 : [test] Marking packed-elements test as slow on PPC and S390 simulator

Change-Id: I7ae01e67ade94530909914df97a17684f8c277f7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1606281Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#61443}

test/mjsunit/mjsunit.status

@@ -883,6 +883,7 @@
   'regress/regress-crbug-482998': [PASS, SLOW],
   'regress/regress-91008': [PASS, SLOW],
   'regress/regress-779407': [PASS, SLOW],
+  'packed-elements': [PASS, SLOW],
   'harmony/regexp-property-lu-ui': [PASS, SLOW],
   'whitespaces': [PASS, SLOW],
   'generated-transition-stub': [PASS, SLOW],