PPC/S390 : [test] Marking generated-transition-stub test as slow on PPC and S390 simulator

Change-Id: Ie9606382c19723c3eae935b463d1bd31e52529fd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1601424Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#61375}

test/mjsunit/mjsunit.status

@@ -885,6 +885,7 @@
   'regress/regress-779407': [PASS, SLOW],
   'harmony/regexp-property-lu-ui': [PASS, SLOW],
   'whitespaces': [PASS, SLOW],
+  'generated-transition-stub': [PASS, SLOW],
   'wasm/atomics-stress': [SKIP],
   'wasm/atomics64-stress': [SKIP],
 }],  # 'simulator_run and (arch in [ppc64, s390x])'