S390: Skip slow test.

TBR=jyan@ca.ibm.com
NOTRY=true

Review URL: https://codereview.chromium.org/1871893002

Cr-Commit-Position: refs/heads/master@{#35361}

test/webkit/webkit.status

@@ -70,6 +70,10 @@
   # Too slow.
   'dfg-int-overflow-in-loop': [SKIP],
 }], # 'arch == ppc or arch == ppc64'
+['arch == s390 or arch == s390x', {
+  # Too slow.
+  'dfg-int-overflow-in-loop': [SKIP],
+}],  # 'arch == s390 or arch == s390x'
 
 ##############################################################################
 ['asan == True', {