[wasm] Flag atomics tests as slow on ARM64

Change-Id: I960bd425e5ebd4cda1c44c6a6f085b1553d01a29
Reviewed-on: https://chromium-review.googlesource.com/660404Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47955}

test/mjsunit/mjsunit.status

@@ -324,6 +324,7 @@
   'unicodelctest-no-optimization': [PASS, SLOW],
   'unicodelctest': [PASS, SLOW],
   'unicode-test': [PASS, SLOW],
+  'wasm/atomics': [PASS, SLOW],
   'whitespaces': [PASS, SLOW],
 }],  # 'arch == arm64'