PPC/s390x: make atomics(64)-stress as slow test.

Change-Id: I273764acb3881a5f682d3cc77e2c8f66bbb7b071
Reviewed-on: https://chromium-review.googlesource.com/c/1318091Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#57251}

test/mjsunit/mjsunit.status

@@ -774,6 +774,8 @@
   'regress/regress-91008': [PASS, SLOW],
   'harmony/regexp-property-lu-ui': [PASS, SLOW],
   'whitespaces': [PASS, SLOW],
+  'wasm/atomics-stress': [PASS, SLOW],
+  'wasm/atomics64-stress': [PASS, SLOW],
 }],  # 'simulator_run and (arch == ppc or arch == ppc64 or arch == s390 or arch == s390x)'
 
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