PPC/S390: skip JSAtomicsMutexTest.Contention test

This test requires shared cage implemented. More details
under https://crrev.com/c/3673354 .

Change-Id: Ib1d39dcc2fd6c8abb4f79bf6b845132c34d2ec8a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3687475Reviewed-by: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Milad Farazmand <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/main@{#80926}

test/unittests/unittests.status

@@ -73,6 +73,12 @@
   'SweeperTest.DiscardingNormalPageMemory': [SKIP],
 }],  # arch == ppc64
 
+##############################################################################
+['no_js_shared_memory', {
+  # Tests that require shared cage implemented.
+  'JSAtomicsMutexTest.Contention': [SKIP],
+}],  # no_js_shared_memory
+
 ##############################################################################
 ['system == windows and asan', {
   # BUG(893437).