[infra] add mjsunit shards for {V8 Linux - arm - sim - debug}

The bot finishes close to our timeout SLO even after the speed-up attempt at
crrev.com/c/1760810. Example failure:
https://ci.chromium.org/p/v8/builders/ci/V8%20Linux%20-%20arm%20-%20sim%20-%20debug/17596

R=solanes@chromium.org,rmcilroy@chromium.org,mslekova@chromium.org,bbudge@chromium.org
TBR=machenbach@chromium.org

Bug: v8:9633
Change-Id: Ia6ea32e7d465466726dcf16e73b7c7f77c385813
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1760822
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Auto-Submit: Tamer Tas <tmrts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63261}

infra/testing/builders.pyl

@@ -1390,7 +1390,7 @@
       'os': 'Ubuntu-16.04',
     },
     'tests': [
-      {'name': 'mjsunit_sp_frame_access'},
+      {'name': 'mjsunit_sp_frame_access', 'shards': 4},
       {'name': 'mozilla'},
       {'name': 'test262'},
       {'name': 'v8testing', 'shards': 8},