PPC/S390 : [test] Marking regress-779407 test as slow on PPC and S390 simulator

Change-Id: Id27308356d1e2fb46e20225e4cab9ccc006abda7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1599248Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#61286}

test/mjsunit/mjsunit.status

@@ -888,6 +888,7 @@
   'regress/regress-crbug-941743': [SKIP],
   'regress/regress-crbug-482998': [PASS, SLOW],
   'regress/regress-91008': [PASS, SLOW],
+  'regress/regress-779407': [PASS, SLOW],
   'harmony/regexp-property-lu-ui': [PASS, SLOW],
   'whitespaces': [PASS, SLOW],
   'wasm/atomics-stress': [SKIP],