Mark mjsunit/wasm/atomics as slow on arm64

It started timing out on trybots after commit
575ec863.

Tbr: gdeepti@chromium.org
Change-Id: Iab025ccbce15a6c9f978b737f064695d11704c93
Reviewed-on: https://chromium-review.googlesource.com/602990Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47175}

test/mjsunit/mjsunit.status

@@ -321,6 +321,7 @@
   'unicodelctest-no-optimization': [PASS, SLOW],
   'unicodelctest': [PASS, SLOW],
   'unicode-test': [PASS, SLOW],
+  'wasm/atomics': [PASS, SLOW],
   'whitespaces': [PASS, SLOW],
 }],  # 'arch == arm64'